In between timespans Ardour disables freewheeling and
only later resumes freewheel. It can happen that
Session::process can summon the butler while start_audio_export()
calls Track::seek.
Just waiting for an earlier Butler::wait_until_finished() does
not prevent the butler from running again later.
This can lead to concurrent calls of DR::refill_audio from both
the butler and start_audio_export(), leading to garbled buffers.
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