WIP: MutablePolygon - linked list based polygon implementation

allowing rapid insertion and removal of points.
WIP: porting smooth_outward() from Cura.

src/libslic3r/CMakeLists.txt

@@ -145,6 +145,8 @@ add_library(libslic3r STATIC
     Point.hpp
     Polygon.cpp
     Polygon.hpp
+    MutablePolygon.cpp
+    MutablePolygon.hpp
     PolygonTrimmer.cpp
     PolygonTrimmer.hpp
     Polyline.cpp