Hardware GL fog support

[zturtleman]

r_useGlFog 1 is intended to use OpenGL fog instead of software fog. Maybe performance using GL fog will be better for some hardware?

It is based on code from RTCW's global fog, though in Spearmint it is used for fog volumes too.

OpenGL1 renderer:
exp global fog: everything is gray.
linear global fog + r_vertexLight 1: no issues (other than ones caused by vextexlight)?
linear global fog + r_vertexLight 0: issues on multistage shaders.

( Tested using ET's battery map, +set fs_game etmain +devmap battery )

OpenGL2 renderer:
there is some code for r_useGlFog, but it isn't run. Need to port code over from GL1.

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It's buggy and currently doesn't have clear benefits over current fog method. Not a high priority to get working.