Introduction of ClipperLib_Z: The Clipper library compiled with support

of the Z coordinate, compiled in the ClipperLib_Z namespace.

Update of Lukas's new brim clipping:
All the brim contours are now clipped by the ClipperLib_Z library
in one shot.

src/clipper/CMakeLists.txt

@@ -4,4 +4,6 @@ cmake_minimum_required(VERSION 2.6)
 add_library(clipper STATIC
     clipper.cpp
     clipper.hpp
+    clipper_z.cpp
+    clipper_z.hpp
 )