mesa: Return 8 bits for GL_TEXTURE_RED_SIZE on RGTC formats.

mesa: Return 8 bits for GL_TEXTURE_RED_SIZE on RGTC formats.

From the issues section of the GL_ARB_texture_compression_rgtc extension:

15) What should glGetTexLevelParameter return for
    GL_TEXTURE_GREEN_SIZE and GL_TEXTURE_BLUE_SIZE for the RGTC1
    formats?  What should glGetTexLevelParameter return for
    GL_TEXTURE_BLUE_SIZE for the RGTC2 formats?

    RESOLVED:  Zero bits.

    These formats always return 0.0 for these respective components
    and have no bits devoted to these components.

    Returning 8 bits for red size of RGTC1 and the red and green
    sizes of RGTC2 makes sense because that's the maximum potential
    precision for the uncompressed texels.

Thus, we need to return 8 bits for GL_TEXTURE_RED_SIZE on all RGTC formats
and 8 bits for GL_TEXTURE_GREEN_SIZE on RGTC2 formats.  BLUE should be 0.

Fixes oglconform/rgtc/advanced.texture_fetch.tex_param.

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Brian Paul <brianp@vmware.com>