compositor, clients: make damage double-buffered

compositor, clients: make damage double-buffered

This change depends on the Wayland commit
"protocol: double-buffered state for wl_surface".

Implement double-buffering of damage in the compositor as required by
the new protocol.

Ensure all Weston demo clients call wl_surface_commit() after
wl_surface_damage().

Mesa does not need a fix for this, as the patch adding
wl_surface_commit() call to Mesa already takes care of damage, too;
Mesa commit: "wayland: use wl_surface_commit()"

Signed-off-by: Pekka Paalanen <ppaalanen@gmail.com>