Fix various bugs when using JACK (#4005)

* Fix crash on closing
* Fix audio rendering artifacts
* Make LMMS work properly after rendering

include/AudioJack.h

@@ -37,6 +37,7 @@
 #include <QtCore/QVector>
 #include <QtCore/QList>
 #include <QtCore/QMap>
+#include <QMutexLocker>
 
 #include "AudioDevice.h"
 #include "AudioDeviceSetupWidget.h"
@@ -107,6 +108,7 @@ private:
 
 	bool m_active;
 	bool m_stopped;
+	QMutex m_processingMutex;
 
 	MidiJack *m_midiClient;
 	QVector<jack_port_t *> m_outputPorts;