Update for multi-polygon compliance with spec v1.0.2

[bcamper]

The upcoming spec v1.0.2 clarifies how multi-polygons should be encoded: all polygons should be "flattened" into one series of rings, with the winding order of the rings used to determine which are interior/exterior. An exterior ring indicates the start of a new polygon.

I believe this differs from our current handling of multi-polygons, where we "exploded" each polygon into its own feature, and copied the properties of each (as there was no official handling of multi-polygons previously). We should update to be compliant. Tangram already handles the new multi-polygon encoding so no client changes are needed on that side (style rules should be compatible with either method).

mapbox/vector-tile-spec#36

[nvkelso]

@bcamper Will this change work with older v0.3 and earlier Tangrams or does it require the new v0.4 release?

[bcamper]

We should test it but as far as I am aware it should not require any Tangram changes. For decoding MVT tiles we use Mapbox's own JS library. Related, for non-tiled datasources, we do the same poly/ring detection on data generated by Mapbox's geojson-vt (which also outputs multi-polygons in this flattened format), see https://github.com/tangrams/tangram/blob/master/src/sources/geojson.js#L159.

[rmarianski]

This fix didn't end up getting deployed with the latest update. I added tickets to the appropriate chef repos to ensure we get the latest version of mapbox-vector-tile deployed in the future.

tilezen/chef-tilequeue#12
tilezen/chef-tileserver#1

[nvkelso]

Once deployed, how can we verify the fix?

[bcamper]

We should identify a few multipolygon features and compare before/after to make sure they have not changed. I believe The Lake in Central Park is one (we had issues with it previously). Any good candidates?