Remove the color attachment completely in Metal and Vulkan clips.
When the color write mask is set to all false, Metal is smart about not
executing the fragment shader at all, but some Vulkan and GL drivers may
not be. GL requires binding shader objects to draw, and so a cap check
is needed here.
In the future, we could add minimal no-op shaders to use with the GL
clip pipeline to shave off a few flops on drivers that aren't smart
about the color write mask.
6bb9c99a11