mesa: Don't choose S3TC for generic compression if we can't compress.

mesa: Don't choose S3TC for generic compression if we can't compress.

If the app is asking us to do GL_COMPRESSED_RGBA, then the app obviously
doesn't have pre-compressed data to hand us.  So don't choose a storage
format that we won't actually be able to compress and store.

Fixes black screen in warzone2100 when libtxc_dxtn is not present.  Also
66 piglit tests.

NOTE: This is a candidate for the 9.2 branch.
Reported-by: Paul Wise <pabs@debian.org>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>