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.

!538