Get rid of invalidate_maybe_recurse and process_updates. Implement
_gdk_windowing_{before,after}_process_all_updates(), and keep track of
when we're inside process_all_updates in the common code so we know
when to flush windows. Implement
_gdk_windowing_window_process_updates_recurse by means of
setNeedsDisplayInRect: displayIfNeeded. Use the added window argument
in begin_paint_region to get the right window (the paintable is always
the impl window now).