vtable updates for glx to remove

Hey folks,

I've been making good progress on getting mesa updated to include =
support for both AppleGL and Gallium backends and switching at runtime.  =
This is mainly in support of a solution for =
https://github.com/XQuartz/XQuartz/issues/446 and is general code =
hygiene that I've wanted to do for over a decade now.

I've merged a number of changes over the past week (misc memory leaks =
identified, macro cleanup, and some AppleGL adoption of existing vtable =
entries).  This next pull request I'm peeling off is probably the one =
that is the most impactful to other platforms as it adds a new vtable =
and changes an existing one.  There were some design decisions made that =
others might feel strongly about (eg: I decided to have a default =
implementation for the new vtable to avoid making changes to all other =
backends), so I wanted to send a note to the list soliciting review =
before merging.

Please see =
https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/42551

Thanks,
Jeremy=