When compiling with SDL_opengl, all kinds of differences between LibGL
and OpenGL constants and types popped up as redefinition warnings and
errors.
This fixes all LibGL-related warnings when compiling PrBoom+ :^)
This commit implements glClipPlane and its supporting calls, backed
by new support for user-defined clip planes in the software GPU clipper.
This fixes some visual bugs seen in the Quake III port, in which mirrors
would only reflect correctly from close distances.
We were splitting these API wrappers up into different files without a
quantifiable benefit. Now, it's extremely clear where the direct API
implementation lives. :^)
