Keep track of whether a texture has alpha.

This way we know what sorts of blending we even have to do.  Commonly,
textures don't have any real alpha.

Not yet used for anything.
Ignore texfunc RGBA when texture is full alpha.

This allows us to skip some work in the fragment shader.
Clean up some comments.
Merge pull request #1727 from unknownbrackets/perf

Keep track of whether textures have alpha values or not