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/Fill/FillConcentric.cpp

@@ -33,7 +33,7 @@ void FillConcentric::_fill_surface_single(
 
     // generate paths from the outermost to the innermost, to avoid
     // adhesion problems of the first central tiny loops
-    loops = union_pt_chained_outside_in(loops, false);
+    loops = union_pt_chained_outside_in(loops);
     
     // split paths using a nearest neighbor search
     size_t iPathFirst = polylines_out.size();