Infer MultiPolygon structure from winding order for toGeoJSON

[mourner]

MultiPolygons are flattened in VT, so we need to infer their structure from winding order like this https://github.com/mapbox/mapbox-gl-js/blob/earcut-final/js/util/classify_rings.js, otherwise MultiPolygons are broken (encoded as Polygons, with additional outer rings becoming holes). cc @jfirebaugh

[jfirebaugh]

How about changing loadGeometry() instead? We could have it do classification if type == 3, and return geometry with one more level of array nesting. This would require a major version change for vector-tile-js, but would allow us to remove classify_rings.js from gl-js, rather than duplicating it in both modules.

Then toGeoJSON could inherit the classification behavior, and check the number of polygons to determine whether to use "type": "Polygon" or "type": "MultiPolygon".

[mourner]

@jfirebaugh sounds good to me!

[jfirebaugh]

⇢ #33