backends: Check for updates in the last used device after dispatching events

Instead of relying in the device being updated from different parts of our
machinery for different backends, hook this up to our own event dispatching.
This will allow dropping all other places where this is done.

Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/285>

src/backends/meta-backend.c

@@ -998,6 +998,22 @@ system_bus_gotten_cb (GObject      *object,
                                         NULL);
 }
 
+static void
+update_last_device_from_event (MetaBackend  *backend,
+                               ClutterEvent *event)
+{
+  ClutterInputDevice *source;
+
+  /* Handled elsewhere */
+  if (event->type == CLUTTER_DEVICE_ADDED ||
+      event->type == CLUTTER_DEVICE_REMOVED)
+    return;
+
+  source = clutter_event_get_source_device (event);
+  if (source)
+    meta_backend_update_last_device (backend, source);
+}
+
 /* Mutter is responsible for pulling events off the X queue, so Clutter
  * doesn't need (and shouldn't) run its normal event source which polls
  * the X fd, but we do have to deal with dispatching events that accumulate
@@ -1037,6 +1053,7 @@ clutter_source_dispatch (GSource     *source,
       event->any.stage =
         CLUTTER_STAGE (meta_backend_get_stage (backend_source->backend));
       clutter_do_event (event);
+      update_last_device_from_event (backend_source->backend, event);
       clutter_event_free (event);
     }