Reset port resampler after freewheeling

This clears out any data potently left there by export processing.
Previously this could cause an audible click.

libs/ardour/ardour/audio_port.h

@@ -42,6 +42,9 @@ public:
 	void cycle_end (pframes_t);
 	void cycle_split ();
 
+	/* reset SRC, clear out any state */
+	void reinit ();
+
 	Buffer& get_buffer (pframes_t nframes) {
 		return get_audio_buffer (nframes);
 	}
@@ -53,6 +56,7 @@ protected:
 	friend class PortManager;
 	AudioPort (std::string const &, PortFlags);
 
+
 	/* special access for PortManager only (hah, C++) */
 	Sample* engine_get_whole_audio_buffer ();
 

libs/ardour/ardour/port.h

@@ -121,6 +121,7 @@ public:
 	virtual void cycle_start (pframes_t);
 	virtual void cycle_end (pframes_t) = 0;
 	virtual void cycle_split () = 0;
+	virtual void reinit () {}
 	virtual Buffer& get_buffer (pframes_t nframes) = 0;
 	virtual void flush_buffers (pframes_t /*nframes*/) {}
 	virtual void transport_stopped () {}

libs/ardour/ardour/port_manager.h

@@ -293,6 +293,8 @@ protected:
 	 */
 	void cycle_end (pframes_t nframes, Session* s = 0);
 
+	void reinit ();
+
 	void cycle_end_fade_out (gain_t, gain_t, pframes_t, Session* s = 0);
 
 	static std::string port_info_file ();

libs/ardour/audio_port.cc

@@ -127,6 +127,12 @@ AudioPort::cycle_split ()
 {
 }
 
+void
+AudioPort::reinit ()
+{
+	_src.reset ();
+}
+
 AudioBuffer&
 AudioPort::get_audio_buffer (pframes_t nframes)
 {

libs/ardour/audioengine.cc

@@ -1483,6 +1483,9 @@ AudioEngine::freewheel_callback (bool onoff)
 {
 	DEBUG_TRACE (DEBUG::BackendCallbacks, string_compose (X_("freewheel callback onoff %1\n"), onoff));
 	_freewheeling = onoff;
+	if (!_freewheeling) {
+		PortManager::reinit ();
+	}
 }
 
 void

libs/ardour/port_manager.cc

@@ -1183,6 +1183,13 @@ PortManager::silence (pframes_t nframes, Session* s)
 		}
 	}
 }
+void
+PortManager::reinit ()
+{
+	for (auto const& p : *_ports.reader ()) {
+		p.second->reinit ();
+	}
+}
 
 void
 PortManager::silence_outputs (pframes_t nframes)