Ensure recursing gdk_window_process_all_updates works

Ensure recursing gdk_window_process_all_updates works

There are two issues here. First of all an ignored update didn't
use to unset update_idle which could cause all further idle repaints
to be ignored. (Bug #591583)

Secondly, if we ignore the process_all_updates we may end up not updating
the windows in update_windows unless something else triggers an update.
So, we handle this by checking for recursions and scheduling a new update
at the end of the outermost process_all_updates.