Emit normal_roughness compatibility code in custom functions

[clayjohn]

Fixes: #94777

When reading from the normal_roughness buffer we need to emit compatibility code to ensure that the user gets proper normals even though the texture is stored in a custom format.

In master the compatibility code is only added when the normal_roughness uniform is used directly. When the normal_roughness uniform is passed as an argument to a custom function the compatibility code is not used.

This PR does two things:

1. It checks if the source of the argument is a a screen_texture and emits the compatibility code if it is
2. It bans passing a screen texture to custom function if the custom function is already used with a non-screen texture (we already had this logic in place for when two uniforms were used with different filter or roughness settings)

As a bonus, we can now undo the restriction added in #72300 which forbid passing screen textures to custom functions when using XR shaders.

[BastiaanOlij]

Nice improvement! Looks good to me :)

[akien-mga]

Thanks!