gdk: gdk_window_get_update_area don't remove alpha covered areas

gdk: gdk_window_get_update_area don't remove alpha covered areas

gdk_window_get_update_area is supposed to get the area where things
need painting, and remove them from the update areas. However, if
some area is covered by other windows with an alpha background we
can't just expect whatever the app choses to render in the update
area as correct, so we don't actually remove these areas, meaning
they will get correctly rendered when we get to the expose handlers.