General Polygon Clipper library

A mature 2D polygon package. The wrapping and extension code is free software, but the core gpc library is free for non-commercial usage only. The author says: "This library is provided free of charge for non-commercial users. Reaching its current state took up almost two years of personal development effort. If you have found this software to be useful for your purposes please make a donation and help support the continuation of this project. Thanks!"