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.cpp

@@ -0,0 +1,7 @@
+// Hackish wrapper around the ClipperLib library to compile the Clipper library with the Z support.
+
+// Enable the Z coordinate support.
+#define use_xyz
+
+// and let it compile
+#include "clipper.cpp"