Implement Mapbox custom layer API RFC

[Pessimistress]

For #2132

Background

API proposal to provide a path for deck.gl pure-js and React users to use Mapbox custom layers. This change allows user to leverage deck.gl top-level API such as pickingRadius, onLayerHover etc.

Example:

import Deck, {ScatterplotLayer} from '@deck.gl/core';
import {MapboxLayer} from '@deck.gl/mapbox';

const map = new mapboxgl.Map({...});

const deck = new Deck({
  width: '100%',
  height: '100%',
  longitude: -122.4,
  latitude: 37.8
  layers: [
    new ScatterplotLayer({
	  id: 'my-scatterplot',
	  ...
	})
  ]
});

map.on('load', () => {
  map.addLayer(new MapboxLayer({id: 'my-scatterplot', deck}), getFirstTextLayerId(map.getStyle()));
});

Change List

• Support composite layers in Mapbox integration
• Support Deck top-level props in Mapbox integration

[ibgreen]

Let's discuss the RFC before closing on this.

[ibgreen]

On the surface, this coupling is obviously very undesirable. Having two components that both know about each other breaks encapsulation and usually causes pain down the road.

Part of the problem is calling it deck. Normally it is better to just allow the lower level component (LayerManager in this case) store an 'unspecified' reference to the owning object, that can only be interpreted by higher layers.

Maybe the right approach is to reconsider who owns the context. With the emergence of Deck, ViewManager etc classes we have already removed a lot of functionality out of LayerManager, and I my thinking has been that it should be further "gutted" to just handle layer matching. drawing/picking for instance really doesn't need to be placed in LayerManager

[ibgreen]

Nit: Can we avoid nested functions?

Add a bit more explanation in the header about why this is done (picking...).

[ibgreen]

Call it handlePickingMouseEvent or something that hints at what it does?

[ibgreen]

Storing _deck on the map could avoid having to pass deck around like this...

[Pessimistress]

This is for the use case where we add a MapboxLayer referencing a layer from an existing Deck instance:

new MapboxLayer({id: 'my-layer', deck});