The private function `_clutter_input_device_update()` is not currently
exported.
This function calls `_clutter_input_device_set_actor()` which updates
the `ClutterActor` under the pointer, so making that function available
outside of Clutter will allow to make sure the pointer device actor is
updated prior to do picking.
Also, now that the functions is exported to the upper layers, drop the
underscore suffix from the function name.
https://gitlab.gnome.org/GNOME/mutter/merge_requests/1026
8b3bc1f97d