Visvalingam-Whyatt simplification

[systemed]

This adds the Visvalingam-Whyatt algorithm as an alternative simplification method.

This works by eliminating those points which create the smallest triangular area. It therefore picks up 'spikes' - long, thin protuberances from the line - which Douglas-Peucker would tolerate. It's often used for coastlines and for landuse polygons.

The code is almost exactly @e-n-f's (MIT-licensed) implementation from tippecanoe, based on a Go original by @paulmach. My contribution is just gluing it into tilemaker!

• Unlike our Douglas-Peucker implementation, this isn't yet intersection-aware. That's a todo for a future version. As a substitute it calls make_valid after simplifying a multipolygon, which should catch the most egregious self-intersections.
• It is generally not suitable for roads, especially when combine_below is used. The latter can create V-shaped road polylines, particularly in the case where a dual carriageway merges into a single carriageway. Visvalingam-Whyatt will see that as a spike and eliminate it entirely.
• Performance is broadly on a par with Douglas-Peucker.
• I'd like to add more simplification algorithms in the future, so the simplify_algorithm layer parameter is a string rather than a bool.

Douglas-Peucker airfield:

Visvalingam-Whyatt airfield:

Douglas-Peucker coastline:

Visvalingam-Whyatt coastline - note that the inlets have been simplified away:

Running a Europe extract with the ocean layer set to visvalingam seems to be reliable and not produce any leaky tiles that I've found so far, so I'm inclined to change the OMT-compatible JSON accordingly.