Vulkan: Declare gl_Position as invariant. See #11583

The code that computes shader outputs declared as invariant gets certain
optimizations disabled that could change the value - it makes it more
likely that different ways to do the same calculations end up with the same value.

This can help some types of Z fighting problems and similar.

Generally not a big deal for performance, plus, we're rarely vshader-bottlenecked...

GPU/Common/VertexShaderGenerator.cpp

@@ -276,6 +276,9 @@ bool GenerateVertexShader(const VShaderID &id, char *buffer, const ShaderLanguag
 			// See the fragment shader generator
 			WRITE(p, "layout (location = 3) out highp float v_fogdepth;\n");
 		}
+
+		WRITE(p, "invariant gl_Position;\n");
+
 	} else if (compat.shaderLanguage == HLSL_D3D11 || compat.shaderLanguage == HLSL_D3D9) {
 		// Note: These two share some code after this hellishly large if/else.
 		if (compat.shaderLanguage == HLSL_D3D11) {