Fix frame timings resulting in stuck clients in certain situations

X11 frame timings could get "stuck" if we ended up not drawing a frame when it expect to, due to more aggressive frame drawing avoidance. This is now fixed with more aggressive damage reporting in these situations.

The X11 sync ring could also get into trouble as it expected to be notified about no-op frames. This is now fixed by reporting about "frames" without actual frames (a.k.a. "updates").