The source buffer currently has a thread for each track that feeds the track
with all data in the track buffer until EOS is reached.
Each pass over the track buffer currently waits for the EOS to appear when it's
done iterating the track buffer which is too restrictive.
When the source buffer reaches the end of the track buffer, it should wait for
any new data to be processed -- not just an EOS -- then check for cancellation
if the deadline expires without new data.
Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/8512>
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