Added an alternative Greiner clipping algorithm.

Explored other alternative clipping algorithms and libraries as well.
The Greiner algorithm (written by someone else) actually works, but does
not yet handle coinciding polygon edges (this can be fixed by looking at
Forster's article). Other things is it doesn't take polygons with holes,
or multipolygons.
Added some more dist methods, and played with altern clip and buff algos