message: add layer id to paths by layer
diff: @@ -463,7 +463,11 @@ namespace MakerJs.exporter {
  for (var layer in pathDataByLayer) {
  var pathData = pathDataByLayer[layer].join(' ');
  - createElement("path", { "d": pathData }, layer, null, true);
  + var attrs = { "d": pathData };
  + if (layer.length > 0) {
  + attrs["id"] = layer;
  + }
  + createElement("path", attrs, layer, null, true);
  }
  } else {
  