Fix duplicate Port flush_buffer calls (#9481)

Since 62fc1d3c2e, Delivery buffers were flushed twice.
Once by copy_to_outputs() and again later by
Delivery::flush_buffers. This resulted in duplicate events
during export (see 576840c09e, MIDI buffers are not cleared
after flush to allow export processing to grab the data from
the port-buffers).

The workaround in 62fc1d3c2e is only relevant for ClickIO,
other Deliveries (Send is a Delivery) are explicitly flushed
by Route::flush_processor_buffers_locked.

libs/ardour/ardour/io.h

@@ -193,6 +193,7 @@ public:
 
 	void collect_input (BufferSet& bufs, pframes_t nframes, ChanCount offset);
 	void copy_to_outputs (BufferSet& bufs, DataType type, pframes_t nframes, samplecnt_t offset);
+	void flush_buffers (pframes_t nframes);
 
 	/* AudioTrack::deprecated_use_diskstream_connections() needs these */
 

libs/ardour/delivery.cc

@@ -534,16 +534,10 @@ void
 Delivery::flush_buffers (samplecnt_t nframes)
 {
 	/* io_lock, not taken: function must be called from Session::process() calltree */
-
+	if (_output) {
-	if (!_output) {
+		_output->flush_buffers (nframes);
 		return;
 	}
-
-	PortSet& ports (_output->ports());
-
-	for (PortSet::iterator i = ports.begin(); i != ports.end(); ++i) {
-		i->flush_buffers (nframes);
-	}
 }
 
 void

libs/ardour/io.cc

@@ -1688,9 +1688,13 @@ IO::copy_to_outputs (BufferSet& bufs, DataType type, pframes_t nframes, samplecn
 		port_buffer.read_from (*prev, nframes, offset);
 		++o;
 	}
+}
 
+void
+IO::flush_buffers (pframes_t nframes)
+{
 	/* when port is both externally and internally connected,
-	 * make data directly available to downstream internal ports */
+	 * make data available to downstream internal ports */
 	for (auto const& p : _ports) {
 		p->flush_buffers (nframes);
 	}

libs/ardour/io_plug.cc

@@ -495,9 +495,7 @@ IOPlug::connect_and_run (samplepos_t start, pframes_t n_samples)
 			}
 		}
 	}
-	for (PortSet::iterator i = ports.begin(); i != ports.end(); ++i) {
+	_output->flush_buffers (n_samples);;
-		i->flush_buffers (n_samples);
-	}
 
 	const samplecnt_t l = effective_latency ();
 	if (_plugin_signal_latency != l) {

libs/ardour/session_click.cc

@@ -251,6 +251,7 @@ Session::run_click (samplepos_t start, samplepos_t nframes)
 
 	_click_gain->run (bufs, 0, 0, 1.0, nframes, false);
 	_click_io->copy_to_outputs (bufs, DataType::AUDIO, nframes, 0);
+	_click_io->flush_buffers (nframes);
 }
 
 void