Adapt to changes from these wayland commits:
35fd2a8cc68c42d90756330535de04cbbb4d2613
2bb3ebe1e437acf836449f0a63f3264ad29566f2
f8fc08f77187f6a5723281dab66841e5f3c24320
http://bugzilla.clutter-project.org/show_bug.cgi?id=2474
Always use pageflipping, but avoid full repaint by copying back dirty
regions from front to back. Additionally, we dealy copying back until
we're ready to paint the new frame, so we can avoid copying areas that
will be repainted anyway.
This is the least amount of copying per frame we can get away with at all
and at the same time we don't have to worry about stalling the GPU on
synchronized blits since we always pageflip.
