At a high level, everything that we need to load in sync
with anything else now implements a new SequenceableLoader
class. This includes ChunkTrackStream, since multiple
demuxed tracks in DASH/SS need to be loaded in sync with
one another. At a higher level, SampleSources are also
SequenceableLoaders, which allows them to be kept in sync
by MultiSampleSource.
CompositeSequenceableLoader is able to load multiple
instances SequenceableLoaders in sync with one another,
and is used in various places where this is required.
This change also removes LoadControl registration, which
was complicated and error prone.
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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=126632861
7b9e47ec6b