Enable geometry fade/transparency in the Mobile renderer

This is mostly a copy-paste of similar changes in the Forward+ renderer,
as hinted in godotengine#76774 (comment) and godotengine#87231 (review),
along with a tweak to the shader and pipeline model.

The shader pipeline tweak involves distinguishing geometry instance-enabled
transparency from material-enabled transparency. A material cannot support
transparency on a per-instance level; a separate pipeline must be used when rendering.

Originally in c571e4a this was a non-trivial refactoring in Forward+
(which in the past few years has evolved a few more times to account for the multitude of combinations
allowed there).

I opted to follow a similar sentiment to `SceneShaderForwardClustered::PipelineColorPassFlags`
in a simpler `SceneShaderForwardMobile::PipelinePasses` enum, which only accounts
for don't-care or opaque pipelines vs. color transparency-enabled ones.

This multiplies the number of `PipelineCacheRD`s by two, but otherwise accomplishes
the goals without adding unnecessary changes or shooting myself or others in the foot (I hope).

BTW I also reorganized and removed some redundant checks in
`RenderForwardClustered::_geometry_instance_add_surface_with_material`
(since I was comparing this side-by-side with the `RenderForwardMobile` version, which seems
better-organized). There is no functional change since `has_alpha` is the only variable used
and includes all the checks anyway.

doc/classes/GeometryInstance3D.xml

@@ -66,7 +66,7 @@
 			The transparency applied to the whole geometry (as a multiplier of the materials' existing transparency). [code]0.0[/code] is fully opaque, while [code]1.0[/code] is fully transparent. Values greater than [code]0.0[/code] (exclusive) will force the geometry's materials to go through the transparent pipeline, which is slower to render and can exhibit rendering issues due to incorrect transparency sorting. However, unlike using a transparent material, setting [member transparency] to a value greater than [code]0.0[/code] (exclusive) will [i]not[/i] disable shadow rendering.
 			In spatial shaders, [code]1.0 - transparency[/code] is set as the default value of the [code]ALPHA[/code] built-in.
 			[b]Note:[/b] [member transparency] is clamped between [code]0.0[/code] and [code]1.0[/code], so this property cannot be used to make transparent materials more opaque than they originally are.
-			[b]Note:[/b] Only supported when using the Forward+ rendering method. When using the Mobile or Compatibility rendering method, [member transparency] is ignored and is considered as always being [code]0.0[/code].
+			[b]Note:[/b] Only supported when using the Forward+ or Mobile rendering methods. When using the Compatibility rendering method, [member transparency] is ignored and is considered as always being [code]0.0[/code].
 		</member>
 		<member name="visibility_range_begin" type="float" setter="set_visibility_range_begin" getter="get_visibility_range_begin" default="0.0" keywords="lod_begin, hlod_begin">
 			Starting distance from which the GeometryInstance3D will be visible, taking [member visibility_range_begin_margin] into account as well. The default value of 0 is used to disable the range check.
@@ -131,11 +131,11 @@
 		</constant>
 		<constant name="VISIBILITY_RANGE_FADE_SELF" value="1" enum="VisibilityRangeFadeMode">
 			Will fade-out itself when reaching the limits of its own visibility range. This is slower than [constant VISIBILITY_RANGE_FADE_DISABLED], but it can provide smoother transitions. The fading range is determined by [member visibility_range_begin_margin] and [member visibility_range_end_margin].
-			[b]Note:[/b] Only supported when using the Forward+ rendering method. When using the Mobile or Compatibility rendering method, this mode acts like [constant VISIBILITY_RANGE_FADE_DISABLED] but with hysteresis disabled.
+			[b]Note:[/b] Only supported when using the Forward+ or Mobile rendering methods. When using the Compatibility rendering method, this mode acts like [constant VISIBILITY_RANGE_FADE_DISABLED] but with hysteresis disabled.
 		</constant>
 		<constant name="VISIBILITY_RANGE_FADE_DEPENDENCIES" value="2" enum="VisibilityRangeFadeMode">
 			Will fade-in its visibility dependencies (see [member Node3D.visibility_parent]) when reaching the limits of its own visibility range. This is slower than [constant VISIBILITY_RANGE_FADE_DISABLED], but it can provide smoother transitions. The fading range is determined by [member visibility_range_begin_margin] and [member visibility_range_end_margin].
-			[b]Note:[/b] Only supported when using the Forward+ rendering method. When using the Mobile or Compatibility rendering method, this mode acts like [constant VISIBILITY_RANGE_FADE_DISABLED] but with hysteresis disabled.
+			[b]Note:[/b] Only supported when using the Forward+ or Mobile rendering methods. When using the Compatibility rendering method, this mode acts like [constant VISIBILITY_RANGE_FADE_DISABLED] but with hysteresis disabled.
 		</constant>
 	</constants>
 </class>

scene/3d/visual_instance_3d.cpp

@@ -511,12 +511,12 @@ PackedStringArray GeometryInstance3D::get_configuration_warnings() const {
 		warnings.push_back(RTR("The GeometryInstance3D is configured to fade out smoothly over distance, but the fade transition distance is set to 0.\nTo resolve this, increase Visibility Range End Margin above 0."));
 	}
 
-	if (!Math::is_zero_approx(transparency) && OS::get_singleton()->get_current_rendering_method() != "forward_plus") {
-		warnings.push_back(RTR("GeometryInstance3D transparency is only available when using the Forward+ rendering method."));
+	if (!Math::is_zero_approx(transparency) && OS::get_singleton()->get_current_rendering_method() == "gl_compatibility") {
+		warnings.push_back(RTR("GeometryInstance3D transparency is only available when using the Forward+ or Mobile rendering methods."));
 	}
 
-	if ((visibility_range_fade_mode == VISIBILITY_RANGE_FADE_SELF || visibility_range_fade_mode == VISIBILITY_RANGE_FADE_DEPENDENCIES) && OS::get_singleton()->get_current_rendering_method() != "forward_plus") {
-		warnings.push_back(RTR("GeometryInstance3D visibility range transparency fade is only available when using the Forward+ rendering method."));
+	if ((visibility_range_fade_mode == VISIBILITY_RANGE_FADE_SELF || visibility_range_fade_mode == VISIBILITY_RANGE_FADE_DEPENDENCIES) && OS::get_singleton()->get_current_rendering_method() == "gl_compatibility") {
+		warnings.push_back(RTR("GeometryInstance3D visibility range transparency fade is only available when using the Forward+ or Mobile rendering methods."));
 	}
 
 	return warnings;

servers/rendering/renderer_rd/forward_mobile/render_forward_mobile.cpp

@@ -1921,6 +1921,20 @@ void RenderForwardMobile::_fill_render_list(RenderListType p_render_list, const
 
 		bool uses_lightmap = false;
 		// bool uses_gi = false;
+		float fade_alpha = 1.0;
+
+		if (inst->fade_near || inst->fade_far) {
+			float fade_dist = inst->transform.origin.distance_to(p_render_data->scene_data->cam_transform.origin);
+			// Use `smoothstep()` to make opacity changes more gradual and less noticeable to the player.
+			if (inst->fade_far && fade_dist > inst->fade_far_begin) {
+				fade_alpha = Math::smoothstep(0.0f, 1.0f, 1.0f - (fade_dist - inst->fade_far_begin) / (inst->fade_far_end - inst->fade_far_begin));
+			} else if (inst->fade_near && fade_dist < inst->fade_near_end) {
+				fade_alpha = Math::smoothstep(0.0f, 1.0f, (fade_dist - inst->fade_near_begin) / (inst->fade_near_end - inst->fade_near_begin));
+			}
+		}
+
+		fade_alpha *= inst->force_alpha * inst->parent_fade_alpha;
+		flags |= (uint32_t(fade_alpha * 255.0) << INSTANCE_DATA_FLAGS_FADE_SHIFT) & INSTANCE_DATA_FLAGS_FADE_MASK;
 
 		if (p_render_list == RENDER_LIST_OPAQUE) {
 			if (inst->lightmap_instance.is_valid()) {
@@ -2014,6 +2028,11 @@ void RenderForwardMobile::_fill_render_list(RenderListType p_render_list, const
 #else
 				bool force_alpha = false;
 #endif
+
+				if (fade_alpha < 0.999) {
+					force_alpha = true;
+				}
+
 				if (!force_alpha && (surf->flags & GeometryInstanceSurfaceDataCache::FLAG_PASS_OPAQUE)) {
 					rl->add_element(surf);
 				}

servers/rendering/renderer_rd/forward_mobile/render_forward_mobile.h

@@ -375,6 +375,8 @@ class RenderForwardMobile : public RendererSceneRenderRD {
 		INSTANCE_DATA_FLAG_MULTIMESH_HAS_CUSTOM_DATA = 1 << 15,
 		INSTANCE_DATA_FLAGS_PARTICLE_TRAIL_SHIFT = 16,
 		INSTANCE_DATA_FLAGS_PARTICLE_TRAIL_MASK = 0xFF,
+		INSTANCE_DATA_FLAGS_FADE_SHIFT = 24,
+		INSTANCE_DATA_FLAGS_FADE_MASK = 0xFFUL << INSTANCE_DATA_FLAGS_FADE_SHIFT
 	};
 
 	struct GeometryInstanceLightmapSH {

servers/rendering/renderer_rd/shaders/forward_mobile/scene_forward_mobile.glsl

@@ -912,7 +912,7 @@ void main() {
 	float ao = 1.0;
 	float ao_light_affect = 0.0;
 
-	float alpha = 1.0;
+	float alpha = float((instances.data[draw_call.instance_index].flags >> INSTANCE_FLAGS_FADE_SHIFT) & INSTANCE_FLAGS_FADE_MASK) / float(255.0);
 
 #if defined(TANGENT_USED) || defined(NORMAL_MAP_USED) || defined(LIGHT_ANISOTROPY_USED)
 	vec3 binormal = normalize(binormal_interp);

servers/rendering/renderer_rd/shaders/forward_mobile/scene_forward_mobile_inc.glsl

@@ -162,8 +162,10 @@ layout(set = 0, binding = 2) uniform sampler shadow_sampler;
 #define INSTANCE_FLAGS_MULTIMESH_HAS_COLOR (1 << 14)
 #define INSTANCE_FLAGS_MULTIMESH_HAS_CUSTOM_DATA (1 << 15)
 #define INSTANCE_FLAGS_PARTICLE_TRAIL_SHIFT 16
+#define INSTANCE_FLAGS_FADE_SHIFT 24
 //3 bits of stride
 #define INSTANCE_FLAGS_PARTICLE_TRAIL_MASK 0xFF
+#define INSTANCE_FLAGS_FADE_MASK 0xFF
 
 layout(set = 0, binding = 3, std430) restrict readonly buffer OmniLights {
 	LightData data[];