Clipper2 support

https://sourceforge.net/projects/polyclipping/:
> This library is now obsolete and no longer being maintained.
> It has been superceded by my Clipper2 library - https://github.com/AngusJohnson/Clipper2

Benchmark comparison of Clipper1 and Clipper2 performance: https://www.angusj.com/clipper2/Docs/Changes.htm