glsl: split gl_CurrentAttribFragMESA into elements

This reduces the constant buffer size by eliminating unused elements
because it's no longer a uniform array that the compiler can't split.

This looks silly, but there is no other way because all elements must be
globally declared, which means they can't be generated by a loop.

Reviewed-by: Eric Anholt <eric@anholt.net>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/8183>
2 files changed