Allow SCREEN_UV to be used in light function of spatial shader

[Chaosus]

• Closes Add the built-in SCREEN_UV to the spatial shader's light() function godot-proposals#10222

[Calinou]

Tested locally (rebased on top of master 3978628), it works as expected in all rendering methods.

Testing project: test_pr_94981.zip

One concern though is that this increases the number of shader versions compiled whenever the feature is used, which didn't occur with the workaround described in godotengine/godot-proposals#10222. This has a performance impact.

[Chaosus]

This has a performance impact.

Yeah, maybe its not good. At least there is a simple workaround to use varying:

shader_type spatial;

varying vec2 screen_uv;

void fragment() {
	screen_uv = SCREEN_UV;
}

void light() {
	DIFFUSE_LIGHT = vec3(screen_uv, 0.0);
}

[Chaosus]

@Calinou I've removed the #ifdef statements so it should not generate a shader versions. Check again to ensure that.

[clayjohn]

Looks great!

[Repiteo]

Thanks!