Bugfix: crash in some circumstances when avoid_crossing_perimeters is enabled. #2266

xs/xsp/MotionPlanner.xsp

@@ -8,7 +8,8 @@
 %name{Slic3r::MotionPlanner} class MotionPlanner {
     MotionPlanner(ExPolygons islands);
     ~MotionPlanner();
-
+    
+    int islands_count();
     Polyline* shortest_path(Point* from, Point* to)
         %code%{ RETVAL = new Polyline(); THIS->shortest_path(*from, *to, RETVAL); %};
 };