Manually update toplevel_under_pointer when a grab changes to owner_events

Manually update toplevel_under_pointer when a grab changes to owner_events

When we ungrab the pointer we don't get enter events for the window the
pointer is in at the time of the ungrab, so we manually query for the
window the pointer is in. The same thing actually happens on re-grab if
the previous grab was !owner_events (meaning we don't get crossing events
for windows other than the grab) but the new grab is owner_events (and
thus non-grab windows need to get crossing events).

This factors out some common code and enables it also for the re-grab
to owner_events case.