vnd has already verified that the context tag is valid before this gets
called, and we only set the context tag private data to non-null for
indirect clients. Mesa happens to be buggy and doesn't send MakeCurrent
requests nearly as much as it should for direct contexts, but if you fix
that, then unbinding a direct context would fail here with
GLXBadContextTag.
Sadly Mesa will still need to carry a workaround here for broken
servers, but we should still fix the server.
(cherry picked from commit 95dcc81cb1)
045add8492