Fix MIDI auditioning

The Auditioner is not part of the session route-list and the
auditioner route's I/O latency is never updated.
Session::process_audition() does not handle pre-roll either,
so it need to be zeroed, otherwise Route::roll skips samples.

This has lead to initial samples being skipped, IFF the
auditioner's output-port had non-zero latency.

Since private port-latencies are usually only set for routes
in the route-list, and _remaining_latency_preroll is reset
at transport-stop, this *usually* worked...

Last but not least, MIDI notes need to be resolved when seeking.

libs/ardour/ardour/auditioner.h

@@ -91,6 +91,8 @@ public:
 
 	void set_audition_synth_info(PluginInfoPtr in) { audition_synth_info = in; }
 
+	samplecnt_t output_latency () const { return 0; }
+
 private:
   
 	PluginInfoPtr audition_synth_info;  //we will use this to create a new synth on-the-fly each time an audition is requested

libs/ardour/auditioner.cc

@@ -440,7 +440,12 @@ Auditioner::play_audition (samplecnt_t nframes)
 		_seek_complete = false;
 		_seeking = false;
 		_seek_sample = -1;
-		_disk_reader->reset_tracker();
+		if (_midi_audition) {
+			/* Force MIDI note tracker to resolve any notes that are
+			 * still playing -> set DR::run_must_resolve */
+			_disk_reader->set_pending_overwrite (PlaylistModified);
+			_disk_reader->overwrite_existing_buffers ();
+		}
 	}
 
 	if(!_seeking) {

libs/ardour/session_transport.cc

@@ -695,6 +695,7 @@ Session::butler_completed_transport_work ()
 
 	if (ptw & PostTransportAudition) {
 		if (auditioner && auditioner->auditioning()) {
+			_remaining_latency_preroll = 0;
 			process_function = &Session::process_audition;
 		} else {
 			process_function = &Session::process_with_events;