cad/Clipper2: add A Polygon Clipping and Offsetting library (in C++, C# & Delphi)

It will be used by openscad.

Clipper2:
The Clipper2 library performs intersection, union, difference and
XOR boolean operations on both simple and complex polygons.
It also performs polygon offsetting.
This is a major update of my original Clipper library
that was written over 10 years ago.
That library I'm now calling Clipper1,
and while it still works very well,
Clipper2 is better in just about every way.