audio: moved output to its own high priority thread

macOS audio playback failed due to audio being processed in the main thread.
Now all audio playback has been placed into its own high priority thread to
always keep up better with the operating system's demands.

app/audio/outputmanager.h

@@ -40,12 +40,13 @@ public:
 
   virtual ~AudioOutputManager() override;
 
-  bool OutputIsSet();
-
-  void SetOutputDevice(QAudioDeviceInfo info, QAudioFormat format);
-
+  // Thread-safe
   void Push(const QByteArray &samples);
 
+public slots:
+  // Queued
+  void SetOutputDevice(QAudioDeviceInfo info, QAudioFormat format);
+
   /**
    * @brief Connect a QIODevice (e.g. QFile) to start sending to the audio output
    *
@@ -54,12 +55,14 @@ public:
    */
   void PullFromDevice(const QString &filename, qint64 offset, int playback_speed);
 
+  // Queued
   void ResetToPushMode();
 
-  void SetParameters(const AudioRenderingParams& params);
-
-public slots:
+  // Queued
+  void SetParameters(OLIVE_NAMESPACE::AudioRenderingParams params);
 
+  // Queued
+  void Close();
 
 signals:
   void OutputNotified();
@@ -68,8 +71,9 @@ private:
   QAudioOutput* output_;
   QIODevice* push_device_;
 
-  QByteArray pushed_samples_;
-  int pushed_sample_index_;
+  QMutex push_sample_lock_;
+  QByteArray push_samples_;
+  int push_sample_index_;
 
   AudioOutputDeviceProxy device_proxy_;