backends: Disconnect MetaInputMapper signal connection on device dispose

This signal may be left dangling when disconnecting a device, and be executed
later on if the device is connected again, and mapped to other output. Make it
sure the signal handler is disconnected when unplugging the device.

Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/1657>

src/backends/meta-input-mapper.c

@@ -189,6 +189,7 @@ mapper_input_info_new (ClutterInputDevice *device,
 static void
 mapper_input_info_free (MetaMapperInputInfo *info)
 {
+  g_signal_handlers_disconnect_by_func (info->settings, settings_output_changed_cb, info);
   g_object_unref (info->settings);
   g_free (info);
 }