Support an OfflineRegionDefinition defined by an arbitrary shape

[zugaldia]

At this moment we only support an OfflineTilePyramidRegionDefinition defined by a bounding box. We should let developers use arbitrary shapes to match, for example, the shape of a state, province, etc.

/cc: @jfirebaugh @1ec5

[jfirebaugh]

The pieces that we would need to support this are:

• A set of C++ types representing geographic features. Probably strongly GeoJSON influenced so we could reuse these types for featuresAt results and geoJSONLayer.setData(). Could potentially be extracted from geojsonvt-cpp. (cc @springmeyer)
• For each platform, bindings to the above.
• An implementation of tileCover for the above.

[twbell]

Polygons are P1; let's also consider polyline-as-input in this ticket as a P2. The primary use case for the latter is pre-caching tiles along a route.

[1ec5]

#5707 and #8012 take different approaches to enabling the developer to define an arbitrary shape for offline download.

[1ec5]

#11447 adds mbgl and Android support for offline regions that cover non-pyramidal geometries.

[1ec5]

Fixed in #11447. Thanks @ivovandongen!