code: function serve(server, req, callback) {
  		source.getTile(req.z, req.x, req.y, function(err, buffer) {
  			if (err) return callback(err);
  			var headers = {'Content-Type': 'application/x-protobuf'};
  			if (options.compression === 'gzip') {
  				headers['Content-Encoding'] = 'gzip';
  			}
  			callback(null, buffer, headers);
  		});
  	}
docs: Renders a tile and returns the result as a buffer (PNG),
  plus the headers that should accompany it.
  
  @param {TileServer} server
  @param {TileRequest} req
  @param {function} callback(err, buffer, headers)
  @return {void}