st/mesa: try to find the format matching format+type in decompressed_with_blit

st/mesa: try to find the format matching format+type in decompressed_with_blit

There was the fast path based on _mesa_format_matches_format_and_type
for GetTexImage, but it never worked, because the Mesa format we were testing
there was always compressed. Further testing showed that the fast path
had been completely broken.

In this commit, the somewhat limited helper util_create_rgba_texture is
no longer used and instead, custom code for the texture creation is added,
which tries to find the best matching RGBA8 format, so that we can hit
the fast path *always* if the read format is a variant of RGBA8 and supported
by the driver.

Reviewed-by: Brian Paul <brianp@vmware.com>