Monitor refresh synchronization

[Po Lu]

I just pushed a complete implementation of extended frame
synchronization on X to master.  This provides synchronization with the
compositing manager, which should then synchronize frame updates with
the monitor refresh, thus eliminating the annoying tearing seen while
playing animated GIFs.

It is only available on the no-toolkit and Xt builds (Motif and Lucid),
and requires a compositing manager supporting the frame synchronization
protocol, such as GNOME Shell or KWin.  To enable it, place:

  Emacs.synchronizeResize: extended

in your X defaults file.  It is recommended that double buffering also
be disabled on the Motif or Lucid builds.

Please give it a try, report any problems you see, and let me know if it
leads to a noticable improvement to graphics tearing and jitter.

Thanks in advance.