WIP: Fixed some compilation issues on OSX, fixed some "override" vs. "virtual"

warnings.

src/libslic3r/Polyline.hpp

@@ -65,7 +65,7 @@ public:
     const Point& last_point() const override { return this->points.back(); }
 
     const Point& leftmost_point() const;
-    virtual Lines lines() const;
+    Lines lines() const override;
     void clip_end(double distance);
     void clip_start(double distance);
     void extend_end(double distance);