Refactored library to support custom web workers

[stuarta0]

Added example with Mapzen Terrarium decoder.

I've integrated this into my commercial project and it works well. In short:

• I separated the code that has the special import string for rollup's web worker handling into it's own mapbox-terrain-provider.ts. This means external code can reference the other *.ts files without needing custom config to handle the "web-worker:..." import string.
• I simplified the export for the distributable, as I no longer need them. You're welcome to add them back in if required. Now it only exports the base MapboxTerrainProvider class.
• I removed the git submodule for martini and instead added it as a dependency.

Usage is as follows:

import { MartiniTerrainProvider } from "lib/terrain-provider";
import { WorkerFarmTerrainDecoder } from "lib/worker/decoder";
import TerrariumWorker from "./mapzen.worker";

const terrainDecoder = new WorkerFarmTerrainDecoder({
  worker: new TerrariumWorker()
});

const terrainProvider = new MartiniTerrainProvider({
  resource: ...,
  decoder: terrainDecoder,
});

Where mapzen.worker.ts is the web worker code, using whichever bundler the consumer desires. In this case, webpack with worker-loader.

[davenquinn]

Hi Stuart, it looks like most of these changes are desirable and improve flexibility without overly complicating the API.

I will note that Martini is currently included as a submodule to support minimum triangle sizes at low zooms, a feature that was left out of the original library. There's an open pull request but I won't want to remove the internal dependency until that is integrated upstream.