@Override
    public boolean test(int x, int y) {
      return bitmap.contains(x * maxTilesAtZoom + y);
    }
Returns the union of tiles covered by {@code a} and {@code b}.

@throws IllegalArgumentException if {@code a} and {@code b} have different zoom levels.
planetiler-core/src/main/java/com/onthegomap/planetiler/render/TiledGeometry.java