XEventsQueued(disp, QueuedAlready) only returns the number of events currently in the queue and
does not flush new events. So this must happen elsewhere and if it does not we
will never see any events. i noticed this bug when removing the call to glXSwapBuffers().
Fix this by using XPending() instead as this flushes new events
if the queue is empty before returning the length.
see https://tronche.com/gui/x/xlib/event-handling/XEventsQueued.html
and https://tronche.com/gui/x/xlib/event-handling/XPending.html
