OpenGL2: Replace R_MipMapsRGB() with faster version.
Don't load external GLSL files by default

External GLSL should probably only be used for development testing,
not released products. The GLSL files are tied to the code, and the
code changes some what often.

Fixes using OpenArena 0.8.8 which has incompatible GLSL files in a pk3.
Don't use -mwindows with Clang on Windows

Thanks to stigmha for pointing out this doesn't work in Windows.
guard against out-of-bounds jump table targets
OpenGL2: Add fourth cascade for sun shadows, and adjust sun shadow cvar defaults.
OpenGL2: Remove R_MipMap() and R_MipMap2(), and fix gamma conversion in R_MipMapsRGB()
OpenGL2: Fix a glsl compile error on old hardware with r_shadowFilter 0.