st/mesa: Add GL_TDFX_texture_compression_FXT1 support

st/mesa: Add GL_TDFX_texture_compression_FXT1 support

Eric recently added PIPE_FORMAT_FXT1_RGB[A] as part of his format
unification work.  This was really most of the work of implementing
the extension.  We just need to handle it in a couple of places and
expose the extension.

v2: Reject the new formats in llvmpipe_is_format_supported to prevent
    crashes because it doesn't know how to handle the new formats.

Reviewed-by: Marek Olšák <marek.olsak@amd.com> [v1]
Reviewed-by: Eric Anholt <eric@anholt.net> [v1]