I’m glad to see that the Polygon package is now used by the popular VPython software to support the creation of the new extrusion objects.

The VPython authors even made a special license agreement that allows the use of GPC (and thus Polygon) free of charge for every purpose as part of the VPython distribution:

Polygon is being distributed subject to the following conditions: This distribution contains code from the GPC Library, and/or code resulting from the use of the GPC Library. This usage has been authorized by The University of Manchester, on the understanding that the GPC-related features are used only in the context of this distribution. It is not permitted to extract the GPC code from the distribution as the basis for commercial exploitation, unless a GPC Commercial Use Licence is obtained from The University of Manchester, contact: http://www.cs.man.ac.uk/~toby/gpc/