Misc: xr-glx-hybrid alias & minor fixes

- Add "xr-glx-hybrid" as an alias of "xr_glx_hybrid". (#163)

- Clear damage history in expose_root() and when root window size
  changes. Unfortunately this doesn't fix #181.
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.
Misc: Add --xrender-sync{,-fence} as configuration file option

- Add --xrender-sync{,-fence} as configuration file option.

- Quit on encountering invalid opacity rule.

- Other small changes.