Adds support for fragmented mp4 with multiple sidx boxes

[anthonybajoua]

Summary:

Addresses Github issue #9373
Implements solution 1 proposed by Google maintainer.

• Added test. Generated test file using below command:

ffmpeg -i short_1080p_videoonly_lowbitrate.mp4 -c copy -movflags frag_keyframe+delay_moov+empty_moov+dash sample_fragmented_seekable_multi_sidx.mp4 (Above movflags flags actually result in an unplayable file on this player until this fix.)

Algorithm/Solution

1. Upon encountering multiple sidx atom in track:
2. For each atom in the track:
   1. If sidx atom then aggregate ChunkIndex into master list, else skip
   2. Merge them all into one large chunk index.
   3. Reset the seek pointer to where it was last
      Retry above on I/O failures.

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[colinkho]

Hey @tonihei @icbaker , would you guys be able to take a look? This would help us maintain a vanilla fMp4 extractor!

[icbaker]

We can take a look once the CLA check is passing.