implemented support for GL_EXT_texture_sRGB to preserve precision if

implemented support for GL_EXT_texture_sRGB to preserve precision if
supported (otherwise it just converts to linear and uses regular texture
formats)
renamed r_texture_convertsRGB_ cvars to r_texture_sRGB_ and broke out
the individual skin textures as separate cvars
added vid.forcetextype which allows particular renderpaths to force
desired texture component layouts (GL_RGBA for GLES, otherwise GL_BGRA)

git-svn-id: svn://svn.icculus.org/twilight/trunk/darkplaces@11045 d7cf8633-e32d-0410-b094-e92efae38249
::stable-branch::merge=3a0aa3feb14ad0af4e0c2fb3408d6cca0132ddcf