This avoid having to guards functions that are
only available in fragment shader stage.
Calling the function inside another stage is still
invalid and will yield a compile error on Metal.
The vulkan and opengl glsl patch need to be modified
per stage to allow the fragment specific function
to be defined.
This is not yet widely used, but a good example is
the change in `film_display_depth_amend`.
Rel #137261
Pull Request: https://projects.blender.org/blender/blender/pulls/138280
8dee08996e