New gamma mode

r_glsl_texturegamma enables new gamma mode to used in Blood Omnicide - a
poor man 16-bit-per-channel rendering. Instead of applying gamma as
postprocess, engine applies it during render passes. This eliminates
screen buffer color banding (as pixel shader operates in floats) and
gives much more precision for high gamma and contrast values, but also
have side effect of blended surfaces being gamma'ed twice
(modulate-blend surfaces and rtlights are not gamma'ed because of
artifacts).  Require OpenGL 2.x and v_glslgamma.