wayland/output: Fix crash on monitor update

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 passing the actual pointer to the xdg-output resource destructor.