function debugSumLayerStats(tiles) {
  var list = {},
    tree = {};
  tiles.filter(function (tile) {
    return tile.debug.layers;
  }).forEach(function (tile) {
    // layer list
    Object.keys(tile.debug.layers.list).forEach(function (layer) {
      var counts = tile.debug.layers.list[layer];
      addLayerDebugEntry(list, layer, counts.features, counts.geoms, counts.styles, counts.base);
    });

    // layer tree
    addDebugLayers(tile.debug.layers.tree, tree);
  });
  return {
    list: list,
    tree: tree
  };
}
Retrieves geometry from a tile according to a data source definition
        Returns an array of objects with:
            layer: source layer name
            geom: GeoJSON FeatureCollection