Improvement: --glx-use-copysubbuffermesa

- GLX backend: Add --glx-use-copysubbuffermesa, to use
  MESA_copy_sub_buffer to do partial screen update. Huge performance
  boost on mesa drivers for partial screen updates, but does not work
  for nvidia-drivers and may break VSync. Automagically overrides
  --glx-copy-from-front.

- Add rect_is_fullscreen() to reuse code. Misc changes.

README.md

@@ -11,6 +11,7 @@ partially doing this out of a desire to learn Xlib.
 
 ## Changes from xcompmgr:
 
+* OpenGL backend (`--backend glx`), in addition to the old X Render backend.
 * __inactive window transparency / dimming__
 * __titlebar/frame transparency__ (specified with `-e`)
 * menu transparency (thanks to Dana)