Need to extend the MapboxHTTPAPI class to support specifying custom map style URLs.

[avi-c]

The current API is using URL schemes that are specific to a few default styles.

[rockarts]

This works: mapView.styleURL = URL(string: "mapbox://styles/username/key")

[jim-martin]

This is possible, but styles copied from https://www.mapbox.com/studio/ aren't in the right format.

Style URLs copied from studio look like this mapbox://styles/jimmartin/cjmidt2lq44ei2rlkvpt1hnhz, but the static API uses {username}/{StyleID}, so removing the mapbox://styles/ section should give a custom style.

We should make it possible to copy/paste from studio, but in the meantime, use:
textureStyle: "jimmartin/cjmidt2lq44ei2rlkvpt1hnhz"
instead of
textureStyle: mapbox://styles/jimmartin/cjmidt2lq44ei2rlkvpt1hnhz