[mapbox] add MapboxOverlay

[Pessimistress]

This enables deck.gl to be used with react-map-gl v7.0 along with native controls. It also greatly simplifies the pure-js use case, whereas before the users were required to implement camera synchronization manually.

The MapboxOverlay implements Mapbox's IControl interface. When used, a deck canvas is inserted into Mapbox's controls container, and Mapbox handles all the user interaction. This works almost identically to the GoogleMapsOverlay class.

Change List

• add MapboxOverlay
• get-started example
• docs

[coveralls]

Coverage decreased (-0.4%) to 82.063% when pulling f85938a on x/mapbox-overlay into 70d55e1 on master.

[felixpalmer]

Great to see that the interface is now the same as the GoogleMapsOverlay, will simplify integrations where both can be supported, or swapped out.

One thing I wonder (as this is for future consideration, not this PR) is how we deal with the interleaved layers vs separate canvas overlay. Right now MapboxOverlay will always render into a separate overlaid canvas, while GoogleMapsOverlay will share a canvas (ignoring the legacy raster maps code path). I wonder if this could be made more consistent, either by:

• Switching MapboxOverlay to share the canvas by using a MapboxLayer under the hood
• Providing a prop in both overlays, e.g. interleaved to let the user decide

[dmitov]

@kylebarron @Pessimistress do you guys need a hand with this MR? I'd be glad to help with getting this merged! ✌️

[Pessimistress]

Thanks @felixpalmer. I created issue #6773 to track follow up works.