mesa: Track a gl_format for the texture buffer format.

mesa: Track a gl_format for the texture buffer format.

There was a function full of unused mappings from the GLenum to
datatype/comps, but that wasn't all the information a driver would
want, which includes the other fields that a gl_format has.  Given
that all the texture buffer formats were represented in gl_format,
just use that as our description.

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