Port to Clipper2
Besides actually being maintained this includes interesting things for us:
- a dedicated clip mode for rectangular areas which is said to be much faster and more stable regarding preserving vertex ordering
- the option to attach custom data to vertices, which would allow the removal of our current external maps for that
- direct support for floating point values, avoiding the currently used conversions
- better performance in general
So far this is just a straight 1:1 port though, MRs to make use of the new features will follow.