Initial work for 3D rendering of toolpaths

xs/src/libslic3r/Line.hpp

@@ -55,6 +55,7 @@ class Linef3
     Linef3() {};
     explicit Linef3(Pointf3 _a, Pointf3 _b): a(_a), b(_b) {};
     Pointf3 intersect_plane(double z) const;
+    void scale(double factor);
     
     #ifdef SLIC3RXS
     void from_SV(SV* line_sv);