- Based on comments from H. Verbeet
- Changed the distinction from .rgba & .xyzw masks to only use .xyzw
  in GLSL shaders.  They are interchangeable, and only served to make
  the trace look more intuitive, but they don't always apply as-is, so
  we'll just leave everything to .xyzw.
- Got rid of the "UseProgramObjectARB(0)" call in drawprim.  If there
  is no shader set on the next primitive, then that primitive will
  call UseProgramObjectARB(0) when it begins to draw.