Metal doesn't support RGB textures and the backend converts them to RGBA textures.
During the conversion missing RGB components should be set to 0 and missing A
component should be set to 1.
In the current implementation this was not the case and A components where also set
to 0.
PR should be backported to 4.2
Pull Request: https://projects.blender.org/blender/blender/pulls/126630
529c720457