[cairo] [RFC] New GLX extension: MESA_multithread_makecurrent

[Eric Anholt]

(sorry about the bad series before this one -- ^C of git-send-email
--compose doesn't DWIM)

I'm happy now with the implementation of this extension, I've got
tests for most of the issues mentioned (the one that's really missing
is the bind-to-new-drawable one, but I don't anticipate issues with
it), and cairo-gl is getting a ~35% performance improvement from it
and is passing cairo's threading tests other than the one that fails
due to non-threading-related issuse.

Where do we go from here with a proposed GLX extension?