Previously, process_responses (the common code for _XReply,
_XReadEvents, and _XEventsQueued) took the current request sequence
number as an argument, and did some highly complicated processing to
attempt to process responses in order across threads.  This didn't
actually work.

Now, process_responses handles responses in order, by adding condition
variables to the list of outstanding requests in
dpy->xcb->pending_requests, and blocking on them when those requests
should get processed to allow _XReply to process them; if actually
called from _XReply, it returns when _XReply's request should get
processed.  _XReply broadcasts on the condition variable after it has
read its reply and re-acquired the display lock.

Another great commit brought to you by Jamey Sharp, Josh Triplett, the
Gobby collaborative text editor, conspy, and ridiculous amounts of SSH
forwarding.

Signed-off-by: Josh Triplett <josh@freedesktop.org>
Signed-off-by: Jamey Sharp <jamey@minilop.net>