support for HiDPI

Nobody has claimed this yet.

The module doesn't seem to works with HiDPI screens like Retina.

Example:
save following under `config.json`, then run `/usr/bin/nodejs /usr/bin/tessera -c . -p 28089`

{

 "/toner": {

   "source": "tilejson+http://staging.tile.stamen.com/toner-background/index.json"

  },

 "/toner-http": {

   "source": "http://toner-elb-3vaew2i23b6d-1666871272.us-east-1.elb.amazonaws.com/toner-background/{z}/{x}/{y}.png"

  }

}

http://127.0.0.1:28089/toner-http/6/11/26@2x.png returns a 512x512 image. Fine.

http://toner-elb-3vaew2i23b6d-1666871272.us-east-1.elb.amazonaws.com/toner-background/6/11/26@2x.png returns a 512x512 image (it's the link described in the tilejson file). Fine.

http://127.0.0.1:28089/toner/6/11/26@2x.png returns a 256x256 image. Wrong :-(.

The solution is probably as simple as integrating the modifications proposed in https://github.com/mapbox/node-tilejson/pull/26/files.

Start by reproducing the behavior with the provided config.json and the /toner and /toner-http @2x URLs. Compare the module with the modifications proposed in the linked node-tilejson pull request. Done means the /toner/6/11/26@2x.png endpoint returns the expected 512x512 image rather than 256x256.

Written by the indexing model from the issue text.