New method to rotate only polygons, not translate.

New free function to_polygons. Whithout this function one needs to construct the ExPolygonCollection, which means a deep copy.

xs/src/libslic3r/ExPolygon.cpp

@@ -52,6 +52,15 @@ ExPolygon::translate(double x, double y)
     }
 }
 
+void
+ExPolygon::rotate(double angle)
+{
+    contour.rotate(angle);
+    for (Polygons::iterator it = holes.begin(); it != holes.end(); ++it) {
+        (*it).rotate(angle);
+    }
+}
+
 void
 ExPolygon::rotate(double angle, const Point &center)
 {