scrolledwindow: Ensure the animation is cancelled on arriving scroll events

When the scrolledwindow receives scroll events, it ensures the timeout to
maybe start the "snap back to edges" animation is reset, but it does nothing
about the animation source. It must be reset just the same, to maybe be
started after the timeout fires up.

gtk/gtkscrolledwindow.c

@@ -3055,6 +3055,7 @@ gtk_scrolled_window_scroll_event (GtkWidget      *widget,
 
   if (handled)
     {
+      gtk_scrolled_window_cancel_deceleration (scrolled_window);
       gtk_scrolled_window_invalidate_overshoot (scrolled_window);
 
       if (priv->scroll_events_overshoot_id)