- Changes for new software rasterizer modules

- Changes for new software rasterizer modules
- Remove support for choosing software fallbacks from core code
- Remove partial fallback code from vbrender.c -- drivers are now
  expected to be able to find a triangle/quad function for every state,
  even if they have to use _swsetup_Triangle or _swsetup_Quad.
- Marked derived variables in the GLcontext struct with a leading
  underscore '_'.