Step one in nixing color bits: nix color bits.

Step two will be to consolidate GLShader and GLShader.Core because the
separation is no longer necessary or useful now that each shader has only one
core.
Consolidated extras into core. Nixed color core vestige.

I decided to leave GLShader.Core around rather than merge it into GLShader
because the lifecycle of the GLShader is longer than the core. A single
GLShader may create and destroy its core multiple times over its lifecycle, and
it's nicer to have the core encapsulate all the bits that come and go rather
than have a bunch of mutable state in GLShader which follows implicit and
non-obvious rules about setting up and tearing down.