seat/x11: Add API to get backend

Will be used instead of going via globals by places that already has a
seat to fetch things from.

Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/2014>

src/backends/x11/meta-seat-x11.c

@@ -1821,6 +1821,12 @@ meta_seat_x11_new (MetaBackend *backend,
   return seat_x11;
 }
 
+MetaBackend *
+meta_seat_x11_get_backend (MetaSeatX11 *seat_x11)
+{
+  return seat_x11->backend;
+}
+
 static ClutterInputDevice *
 get_source_device_checked (MetaSeatX11   *seat,
                            XIDeviceEvent *xev)

src/backends/x11/meta-seat-x11.h

@@ -30,6 +30,9 @@ MetaSeatX11 * meta_seat_x11_new (MetaBackend *backend,
                                  int          opcode,
                                  int          logical_pointer,
                                  int          logical_keyboard);
+
+MetaBackend * meta_seat_x11_get_backend (MetaSeatX11 *seat_x11);
+
 gboolean meta_seat_x11_translate_event (MetaSeatX11  *seat,
 					XEvent       *xevent,
 					ClutterEvent *event);