public Feature setSortKeyDescending(int sortKey) {
      return setSortKey(FeatureGroup.SORT_KEY_MAX + FeatureGroup.SORT_KEY_MIN - sortKey);
    }
Sets the value by which features are sorted from high to low within a layer in the output vector tile.
planetiler-core/src/main/java/com/onthegomap/planetiler/FeatureCollector.java