Prevent the algorithm from generating self-intersecting polygons

[mourner]

In some cases, the algorithm generates self-intersections. We should fix this and guarantee that the resulting concave hull is a simple polygon.

[mourner]

Fixed partially but not quite, full fix in progress — will have to index concave hull segments with rbush too, and check intersections against all segments.

[rhwang201]

@mourner any updates on a full fix for self-intersections?

[mourner]

@rhwang201 did a full fix in b2f27df. Are you experiencing a failing case?

[rhwang201]

@mourner seeing self-intersections in a project that uses concaveman to generate polygons. https://github.com/urbica/galton uses https://github.com/stepankuzmin/node-isochrone, which uses v1.1.1 of concaveman, which includes that commit.

Can reproduce by starting a local Galton server following the "Example" section in the README, and then examining the output of curl http://localhost:4000 --get --data 'lng=-122.139935&lat=37.422744&intervals=15&bufferSize=50&cellWidth=0.5&concavity=3&resolution=20000

[mourner]

Can you set up a self-contained test case (just concaveman, without any additional libraries and servers)?