docs: function getFlatVertexCount(positions, size, startIndex = 0, endIndex) {
    endIndex = endIndex || positions.length;
    if (startIndex >= endIndex) {
      return 0;
    }
    return (
      (isFlatRingClosed(positions, size, startIndex, endIndex) ? 0 : 1) +
      (endIndex - startIndex) / size
    );
  }