mesa: fixes for srgb, new srgb formats

mesa: fixes for srgb, new srgb formats

add some more srgb texture formats, including compressed ones
various fixes relating to srgb formats

issues: _mesa_get_teximage is completely broken for srgb textures,
        both for non-compressed ones (swizzling) and compressed ones
        (shouldn't do standard-to-linear conversion)
        texelFetch function may be broken for little or big endian
        (or both...)