mutter would randomly crash in `send_xdg_output_events()` when changing
the fractional scaling:
wl_resource_post_event ()
zxdg_output_v1_send_logical_size ()
send_xdg_output_events ()
wayland_output_update_for_output ()
meta_wayland_compositor_update_outputs ()
on_monitors_changed ()
g_closure_invoke ()
signal_emit_unlocked_R ()
g_signal_emit_valist ()
_signal_emit ()
meta_monitor_manager_notify_monitors_changed ()
meta_monitor_manager_rebuild ()
This is because the xdg-output resource got freed but wasn't removed
from the list of resources.
Fix this by setting the user data of the xdg-output resource to the
corresponding `MetaWaylandOutput` so that the xdg-output resource
destructor can remove it from the list of resources.
https://gitlab.gnome.org/GNOME/mutter/merge_requests/538
2c1a951b6e