Add option to planar-ize the input graph

[dabreegster]

I've got this working nicely in https://github.com/dabreegster/ltn/blob/fb4e947b75e719bfd01f9f3b5bb3c3680d23508c/backend/src/lib.rs#L94. Where bridges/tunnels cross another road, we can insert more intersections, so that the graph becomes planar, and drawing areas gets much nicer.

It'd be ideal to move the code here, but since the underlying geo operations work in euclidean space, probably also have to copy over the mercator.rs stuff. Or ideally upstream that in geo first.