provide plugins with all available buffers.

e.g. Audio in -> Midi out plugins (on an audio-track).
Midi-buffers do exist in the route but no corresponding
(physical) Input port-buffer exists and ardour crashed.

Now, the plugin itself still only accesses the mapped ports
(ChanMapping), but the mapped buffers are at most
"processor_max_streams" and at least the required
in+out buffers for the plugin.