Update last_window only when needed

Update last_window only when needed

_gtk_tooltip_handle_event, which is called for many events in the GTK+
main loop, calls gtk_tooltip_set_last_window, which keeps a weak
reference to the last window we passed through. If the window being
set is the same than the last one there's really no need to update our
weak reference, so add a check for that and exit early.