Clipper optimization:

1) Removed the already commented-out scaling / unscaling when doing
   "safe offsetting"
2) Removed some of the "safe offsetting" at calls where it never was used.
3) Reworked Clipper & ClipperUtils to pass Polygons / ExPolygons / Surfaces
   as input parameters without conversion to ClipperLib::Paths. This
   should save a lot of memory allocation and copying.
4) Reworked conversions from ClipperLib::Paths & PolyTree to Polygons /
   ExPolygons to use the move operator to avoid many unnecessary allocations.
5) Reworked some "union with safe ofsetting" to "offset_ex", which should
   be cheaper.

src/libslic3r/Polygon.cpp

@@ -70,7 +70,7 @@ double Polygon::area() const
 
 bool Polygon::is_counter_clockwise() const
 {
-    return ClipperLib::Orientation(Slic3rMultiPoint_to_ClipperPath(*this));
+    return ClipperLib::Orientation(this->points);
 }
 
 bool Polygon::is_clockwise() const