This allows a TmpFile pointer to be either a Sync or Async (Threaded)
writer. As result we must be able to handle both RT and non RT processing.
Still, post-processing (normalization and encoding) should always
happen faster than realtime (freewheeling).
Since jack does not allow a client to change to freewheeling from within
the process-callback, the async-writer disk-thread FileFlushed is used
to initiate post-processing.
When flushing the buffers of Delivery processors owned by a Route/Track,
inner deliveries of PortInsert processors were missed since PortInsert
is not a Delivery subclass, but rather owns a Delivery as a private
member. Expose a flush_buffers() for PortInsert and call it too.
This is correct since (external) Send is a Delivery subclass, so that
just makes the send part of inserts behave as external sends do.
Route::no_roll(), Route::roll(), Track::no_roll(), AudioTrack::roll()
and MidiTrack::roll() all had the exact same loop for flushing buffers
of their Delivery processors. That was a lot of replicated code that had
to be kept synchronised by hand. Put that code into a protected method
Route::flush_processor_buffers_locked() which is called instead.
* add a threaded TmpFile Writer
* update API calls to that process_export_fw() can be used as
process_function
The idea is to re-use export infrastructure from normalization:
export to a tmp-file and then encode target formats after that.
