Research for d3-geo’s polyhedra projections; issue #86: we need to recreate projection.clipPolygon() at least for this type of polygon.

Polygon coordinates given by adding these lines in src/polyhedra/index.js:

```
var polygon = [];
outline({point: function(λ, φ) { polygon.push([λ, φ]); }}, root);
polygon.push(polygon[0]);
console.log(polygon);
```

Here we exagerated the value of `epsilon`

to show how this works.

With the usual `epsilon`

, the polygon would look like a line, because its “interior” (in purple) covers the whole sphere. The green line is actually the polygon’s edge on both sides. The polygon has almost no “exterior” (in white).

forked from mbostock‘s block: This Is a Globe

forked from Fil‘s block: Furuti 3 globe clip polygon