Clipper is a "polygon and line clipping and offsetting library" (http://angusj.com/delphi/clipper.php)
- Fast (http://rogue-modron.blogspot.com/2011/04/polygon-clipping-wrapper-benchmark.html)
- Easy to integrate: no external library dependencies; only 2 source files; Boost Software License
- Seems to be actively maintained (http://sourceforge.net/p/polyclipping/code/commit_browser, http://sourceforge.net/p/polyclipping/discussion/)
Ideally, we should design a geometry API first before choosing a backend library. But I feel Clipper suits LOVE well, so I'd like to mention it.
I know such libraries could be used via FFI, but I would like to see more geometry utilities supported by default.