clutter: Pass scroll source to discrete events

This piece of information was lost, always returning
"unknown". We can do better on the native backend.

Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/3642>

src/backends/native/meta-seat-impl.c

@@ -990,6 +990,7 @@ notify_discrete_scroll (ClutterInputDevice     *input_device,
                                        NULL,
                                        modifiers,
                                        GRAPHENE_POINT_INIT (x, y),
+                                       scroll_source,
                                        direction);
 
   queue_event (seat_impl, event);