osmesa: fix renderbuffer format selection

osmesa: fix renderbuffer format selection

The gl_renderbuffer::Format field wasn't always set properly.  This
didn't matter much in the past but with the recent swrast/renderbuffer
mapping changes, core Mesa will be directly touching OSMesa colorbuffers
so using the right MESA_FORMAT_x value is important.

Unfortunately, there aren't MESA_FORMATs for all the possible OSmesa
format/type combinations, such as GL_FLOAT / OSMESA_ARGB.  If anyone
runs into these we can add new Mesa formats.

v2: add warnings for unsupported formats, fix ARGB_REV mix-up.

NOTE: This is a candidate for the 8.0 branch.

Reviewed-by: José Fonseca <jfonseca@vmware.com>