WIP "ensure verticall wall thickness" rework:

1) New region expansion code to propagate wave from a boundary
   of a region inside of it.
2) get_extents() extended with a template attribute to work with
   zero area data sets.
3) ClipperZUtils.hpp for handling Clipper operation with Z coordinate
   (for source contour identification)

src/libslic3r/libslic3r.h

@@ -124,8 +124,7 @@ inline void append(std::vector<T>& dest, std::vector<T>&& src)
         dest.insert(dest.end(),
             std::make_move_iterator(src.begin()),
             std::make_move_iterator(src.end()));
-
-        // Vojta wants back compatibility
+        // Release memory of the source contour now.
         src.clear();
         src.shrink_to_fit();
     }
@@ -161,8 +160,7 @@ inline void append_reversed(std::vector<T>& dest, std::vector<T>&& src)
         dest.insert(dest.end(), 
             std::make_move_iterator(src.rbegin()),
             std::make_move_iterator(src.rend()));
-
-    // Vojta wants back compatibility
+    // Release memory of the source contour now.
     src.clear();
     src.shrink_to_fit();
 }