Add decals_disabled render mode and BaseMaterial3D property

[Calinou]

This adds a decals_disabled render mode that can be used in shaders and toggled in BaseMaterial3D. When this flag is used, decals will not be displayed on the material regardless of the layers its mesh are present on. This may be more convenient or flexible for certain materials that should never receive decals, such as VFX materials or parts of a mesh that use a separate material (such as eyes on a character).

• This closes Add a material flag to set a specific material to not receive decals godot-proposals#9055.

Testing project: test_debug_draw_modes_decals.zip

Preview

The cyan toruses have the Disable Decals property enabled on their material, but are still on the default VisualInstance3D layer. A white torus in the decal's area had its layer changed (with the decal's cull mask being modified) so it doesn't receive the decal.

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[clayjohn]

I don't think we should add a new render mode for this. I think we should disable decals the same way we disable other clustered elements when we use the UNSHADED debug mode. I.e. by ignoring the decal element array:

godot/servers/rendering/renderer_rd/renderer_scene_render_rd.cpp

Lines 1156 to 1160 in 7d2ca2d

	if (get_debug_draw_mode() == RS::VIEWPORT_DEBUG_DRAW_UNSHADED) {
	render_data.lights = ∅
	render_data.reflection_probes = ∅
	render_data.voxel_gi_instances = ∅
	}

I think we can just add:

render_data.decals = ∅

Which saves us a lot of complexity.

[Calinou]

I don't think we should add a new render mode for this. I think we should disable decals the same way we disable other clustered elements when we use the UNSHADED debug mode. I.e. by ignoring the decal element array:

I've done this in #89253. I'll leave this PR open as it addresses an existing proposal.

[Mickeon]

(I do agree with clayjohn by the way, just making sure everything is clean in case of a change of heart)