Referential equality is going to become important for detecting
whether a SampleStream has been replaced, so we need to create
new instances as we do elsewhere.
This also enables multiple SampleStreams to be provided for a
single TrackGroup, as is also true for DASH and SmoothStreaming.
It's forbidden to ask for multiple SampleStreams from a single
TrackGroup currently, but we may choose to relax that at some
point (and indicate whether it's allowed as a flag on each
TrackGroup).
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