Port of Angus Johnson's polygon clipper library to Java

http://www.angusj.com/delphi/clipper.php

Clipper library and this Java port are licensed under Boost
Software Licence ver. 1.0. This license is GPL compatible
and non-viral, so it's perfectly legal to use this code in
GPL-licenced JOSM plugins.

See http://cs.brown.edu/~jwicks/boost/more/license_info.html
or https://www.gnu.org/licenses/license-list.en.html for
more information.
Add new PolygonClipper based on Angus Johnson's Clipper classes
LPIS: use new PolygonClipper