mesa: Replace VersionMajor/VersionMinor with a Version field.

mesa: Replace VersionMajor/VersionMinor with a Version field.

As we get into supporting GL 3.x core, we come across more and more features
of the API that depend on the version number as opposed to just the extension
list.  This will let us more sanely do version checks than "(VersionMajor == 3
&& VersionMinor >= 2) || VersionMajor >= 4".

v2: Fix a bad <= 30 check.

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>