Bug fix #181: Add --xrender-sync{,-fence}

- Add --xrender-sync{,-fence} to deal with redraw lag issue on GLX
  backend. --xrender-sync-fence requires a sufficiently new xorg-server
  and libXext. NO_XSYNC=1 may be used to disable it at compile time.
  Thanks to tchebb for reporting and everybody else for testing. (#181)

- A bit code clean-up. Replace a few XSync() with XFlush() to minimize
  the latency.