wayland: Check whether Xwayland window had a prior surface in generic code

Since commit 8df2a145 (As pointed out by Robert Mader) we just happened do this check when doing the first lookup for a Wayland surface for a XWayland window, when we are later notifying upon surface creation we just set the relation with no further checks.

The cases pointed out in the comment (eg. window changing decoration) might presumably happen in a quick enough sequence that we have two scheduled associations on the fly, so move this check to the more generic meta_xwayland_associate_window_with_surface() which is called on both immediate and delayed paths.

#361 (closed)