Investigate comitting window surfaces at single point from event dispatching

We commit the window surfaces on Wayland today from each QWaylandWindow, so if multiple windows are in use, and modified, there might be a delay between each window's commit, causing the commit to hit the server at different frames.

One way to improve this would be to make the commit in the QWaylandWindow be deferred until returning back to the event loop, where all the windows that need a commit are committed in sequence. 

This would increase the chance that the commits land on the same frame, but not guarantee it. To guarantee it, we would need a commit transaction primitive in the Wayland protocol, but building and testing such a primitive would be a lot easier if we already do the commit from a single point.