Fail less often for intersection geometry

Especially when we use raw OSM footways, we break on so many intersection geometry cases (the circles):

I'll start looking into these, but I've also been looking at how https://github.com/StrandedKitty/streets-gl handles things. It's much more robust.

TODO:

- [ ] Can we have an optional mode where we don't trim back road center-lines at all and have empty intersection polygons, just to debug and see how it looks?
- [ ] Investigate what streets-gl does and see if we can adapt ideas from it

CC @BudgieInWA in case you're interested or have ideas. osm2streets has revived thanks to the Muv work. I'm kind of interested in getting footways and crossings all showing up exactly as they're in OSM, and generally getting rid of the old inferred sidewalks mode.

この issue は、OSM の footway を処理する際に交差部分のジオメトリで発生する失敗に関するものです。道路の中心線をトリミングして交差ポリゴンを作成する osm2streets のコードを確認してください。堅牢性に関するアイデアを得るために、streets-gl リポジトリと比較してください。まず、トリミングを無効にして未加工のジオメトリを可視化できる、オプションのデバッグモードを実装します。

索引モデルが issue の本文から書いたものです。