call SDL_PumpEvents() so the X11 driver sets its this->hidden->switch_waiting,
then set a fullscreen OpenGL window (which makes the X11 driver tear down and
create a new window instead of just resizing the existing one), poll for
events, and the newly-created window will think it needs to pop back to a
window. Fixed by resetting switch_waiting to zero if X11_CreateWindow() had
to tear down a previous window.

--HG--
extra : convert_revision : svn%3Ac70aab31-4412-0410-b14c-859654838e24/trunk%40861