WIP: MutablePolygon - linked list based polygon implementation

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

tests/libslic3r/CMakeLists.txt

@@ -11,6 +11,7 @@ add_executable(${_TEST_NAME}_tests
 	test_geometry.cpp
 	test_placeholder_parser.cpp
 	test_polygon.cpp
+	test_mutable_polygon.cpp
 	test_stl.cpp
 	test_meshsimplify.cpp
 	test_meshboolean.cpp