Many changes and fixes to remove leaks and return objects by reference

xs/src/Polyline.cpp

@@ -8,18 +8,17 @@ Polyline::last_point() const
     return new Point(this->points.back());
 }
 
-Lines
-Polyline::lines()
+void
+Polyline::lines(Lines &lines) const
 {
-    Lines lines;
+    lines.clear();
     for (int i = 0; i < this->points.size()-1; i++) {
         lines.push_back(Line(this->points[i], this->points[i+1]));
     }
-    return lines;
 }
 
 SV*
-Polyline::to_SV_ref() const
+Polyline::to_SV_ref()
 {
     SV* sv = newSV(0);
     sv_setref_pv( sv, "Slic3r::Polyline::Ref", (void*)this );