kms/result-listener: Allow setting a custom main context

While the default when passing NULL will be the main context of the main
thread, make it possible to specify another main context, so that
result handlers can be invoked on the right thread.

Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/2777>

src/backends/native/meta-cursor-renderer-native.c

@@ -361,6 +361,7 @@ assign_cursor_plane (MetaCursorRendererNative *native,
                                                 cursor_hotspot_y);
 
   meta_kms_update_add_result_listener (kms_update,
+                                       NULL,
                                        on_kms_update_result,
                                        native);