Ardour tried to make an educated guess at the initial number of outputs
for a new send. It used the channel configuration of the master bus, if
it existed, else the channel configuration of the route itself.
That guess is good in most cases, but in the case of a track/bus without
audio channels, creating a send with audio doesn't make sense. In that
case, also use the route outputs as a base for the send configuration.
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.
