private static void addAdjacencyEntry(IntObjectMap<IntArrayList> result, int from, int to) {
    IntArrayList ilist = result.get(from);
    if (ilist == null) {
      result.put(from, ilist = new IntArrayList());
    }
    ilist.add(to);
  }
Returns a map from index in {@code geometries} to index of every other geometry within {@code minDist}.
planetiler-core/src/main/java/com/onthegomap/planetiler/FeatureMerge.java