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/libslic3r/pchheader.hpp

@@ -105,6 +105,8 @@
 #include <cereal/access.hpp>
 #include <cereal/types/base_class.hpp>
 
+#include <clipper/clipper_z.hpp>
+#include <clipper/clipper.hpp>
 #include "BoundingBox.hpp"
 #include "ClipperUtils.hpp"
 #include "Config.hpp"