Removed debug message.
Rationalized the division between TexDraw and TexClip a whole lot.

While I find it hard to believe that I would have done all that just by not
thinking, I sure can't think of any Earthly reason why it should exist, and
removing it certainly doesn't seem to have any ill effects either. It even
fixed the alpha-coverage application bug.
Allow modifying the mains of shader contexts with more arbitrary code.
Added the GLSL discard statement.
Added proper GLSL support for tex-clipping.