Fixes for Greiner Hormann clipping

[jimc1664]

Hey, thanks for making this very useful.

I did have some issues with the clipping Greiner Hormann clipping which I think fixed or made less broken.
If you have to time to look over that I can setup a pull request?

Using it to preprocess holes before triangulation, eg:

I can add bits for that too if you want.

-Jim