waylandsink: prevent frame callback being released twice

waylandsink: prevent frame callback being released twice

For those using context from the application which
would be the embedded video case, if the frame callback
is entering at the same time as window is finalizing,
a wayland proxy object would be destroyed twice, leading
the refcout less than zero in the second time, it can
throw an abort() in wayland.

For those top window case, which as a directly connection
to the compositor, they can stop the message queue then
the frame callback won't happen at the same time as the
window is finalizing. It doesn't think it would bother
them about this.

Part-of: <https://gitlab.freedesktop.org/gstreamer/gst-plugins-bad/-/merge_requests/1883>