also scale drag move threshold by UI scale

If UI scale is > 1, pixels are likely small, so it seems appropriate to scale

gtk2_ardour/editor_drag.cc

@@ -319,6 +319,9 @@ Drag::move_threshold () const
 	x *= UIConfiguration::instance().get_drag_sensitivity ();
 	y *= UIConfiguration::instance().get_drag_sensitivity ();
 
+	x *= UIConfiguration::instance().get_ui_scale ();
+	y *= UIConfiguration::instance().get_ui_scale ();
+
 	return std::make_pair (x, y);
 }