dashdemux2: Add matroska parsing

  1. Currently, dashdemux2 will play with fixed bitrate for webm stream and can't change presentation of it according the actual bitrate.
  2. The patch can detect and parse matroska stream. And change presentation index according to the current bitrate after every subfragment.
  3. The test webm stream: https://storage.googleapis.com/shaka-demo-assets/sintel-webm-only/dash.mpd
Edited by elliot chen

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