v2: Rebase on master, notably adding XCB for
X_GLXvop_MakeCurrentReadSGI (anholt).
v3: Fix use-after-free (caught by Julien).
Reviewed-by: Mikhail Gusarov <dottedmag@dottedmag.net> (v1)
Signed-off-by: Julien Cristau <jcristau@debian.org>
Signed-off-by: Eric Anholt <eric@anholt.net>