解析 webm 和 Fragmented mp4 webm 参考 mp4 参考 https://dashif-documents.azurewebsites.net/Guidelines-TimingModel/master/Guidelines-TimingModel.html#addressing-indexed-indexstructure webm 参考 https://github.com/cellar-wg/ebml-specification/blob/master/specification.markdown#variable-size-integer