input-device: Warn if the device has no associated stage

Discarding the event without any warning when the device has no
associated stage makes it hard to find the bug for people implementing
new event backends. We should really warn for that abnormal condition in
_clutter_input_device_update().

clutter/clutter-input-device.c

@@ -513,8 +513,12 @@ _clutter_input_device_update (ClutterInputDevice *device)
   g_return_val_if_fail (device->device_type == CLUTTER_POINTER_DEVICE, NULL);
 
   stage = device->stage;
-  if (stage == NULL)
-    return NULL;
+  if (G_UNLIKELY (stage == NULL))
+    {
+      g_warning ("No stage defined for device '%s'",
+                 clutter_input_device_get_device_name (device));
+      return NULL;
+    }
 
   clutter_input_device_get_device_coords (device, &x, &y);