More work on the clipping method for medial axis. Includes Polyline->is_valid()

xs/xsp/Polyline.xsp

@@ -24,7 +24,9 @@
     Point* last_point()
         %code{% const char* CLASS = "Slic3r::Point"; RETVAL = THIS->last_point(); %};
     double length();
+    bool is_valid();
     void clip_end(double distance);
+    void clip_start(double distance);
 %{
 
 Polyline*