Xwayland can disappear at any time, for example during a new_async() or
read_async() call. When we eventually finalize the stream, the X11
display it was created for is gone, thus can't clean up the X11
resources. Handle this by making the MetaX11Display pointer a weak
pointer, and ignore cleaning up if it disappeared. This is fine since
the X11 server it created those resources one is gone already.
Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/2364>
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