monitor-config-manager: Allow backends to assign extra data

When we're configuring monitors, allow backends to add backend specific
assignments during resource assignment (mapping connectors and CRTCs
etc).

This will later allow the native backend's KMS monitor resources to
assign a primary plane and optionally a cursor plane during
configuration. This will then dictate what plane will be used for
primary plane updates, as well as cursor updates, until reconfigured
again.

Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/3428>

src/backends/meta-monitor-manager-private.h

@@ -87,6 +87,9 @@ struct _MetaCrtcAssignment
   graphene_rect_t layout;
   MetaMonitorTransform transform;
   GPtrArray *outputs;
+
+  gpointer backend_private;
+  GDestroyNotify backend_private_destroy;
 };
 
 /*