Support variable length sgpd boxes in fMPEG

[dimitry-unified-streaming]

For sgpd boxes with version >= 1, the default_length field is zero if the length of the following sample group entries is variable.

Since the FragmentedMp4Extractor only supports one entry, skip the description_length field if applicable (e.g. when version >= 1 and default_length == 0), instead of throwing an error.

For parsing the following CencSampleEncryptionInformationGroupEntry, this should make no difference.

Issue: #3177

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[microkatz]

@dimitry-unified-streaming

Thank you for creating a PR for this feature! Would you please write some unit tests providing content with variable length sgpd boxes?

[google-oss-bot]

Hey @dimitry-unified-streaming. We need more information to resolve this issue but there hasn't been an update in 14 weekdays. I'm marking the issue as stale and if there are no new updates in the next 7 days I will close it automatically.

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