DiskWriter::transport_looped() is called from the session
when engine loops. This does not take local disk-reader run()
latency offset into account.
finish_capture() needs to be postponed until the disk-writer
itself reaches the loop-position. This is achieved by
postponing loop() and calling it once loop-length of samples
has been captured.
This works because engaging loop always seeks to the loop-position
and first loop resets _capture_captured.
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