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/clipper_z.hpp

@@ -0,0 +1,18 @@
+// Hackish wrapper around the ClipperLib library to compile the Clipper library with the Z support.
+
+#ifndef clipper_z_hpp
+#ifdef clipper_hpp
+#error "You should include the clipper_z.hpp first"
+#endif
+
+#define clipper_z_hpp
+
+// Enable the Z coordinate support.
+#define use_xyz
+
+#include "clipper.hpp"
+
+#undef clipper_hpp
+#undef use_xyz
+
+#endif clipper_z_hpp