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/outputdeviceproxy.cpp

@@ -25,10 +25,6 @@
 
 OLIVE_NAMESPACE_ENTER
 
-AudioOutputDeviceProxy::AudioOutputDeviceProxy()
-{
-}
-
 AudioOutputDeviceProxy::~AudioOutputDeviceProxy()
 {
   if (file_.isOpen()) {
@@ -76,31 +72,30 @@ void AudioOutputDeviceProxy::close()
 
 qint64 AudioOutputDeviceProxy::readData(char *data, qint64 maxlen)
 {
-  if (file_.isOpen()) {
-
-    qint64 read_count;
-
-    if (tempo_processor_.IsOpen()) {
-
-      while ((read_count = tempo_processor_.Pull(data, static_cast<int>(maxlen))) == 0) {
-        int dev_read = static_cast<int>(ReverseAwareRead(data, maxlen));
-
-        if (!dev_read) {
-          break;
-        }
-
-        tempo_processor_.Push(data, dev_read);
-      }
-
-    } else {
-      // If we aren't doing any tempo processing, simply passthrough the read signal
-      read_count = ReverseAwareRead(data, maxlen);
-    }
-
-    return read_count;
+  if (!file_.isOpen()) {
+    return 0;
   }
 
-  return 0;
+  qint64 read_count;
+
+  if (tempo_processor_.IsOpen()) {
+
+    while ((read_count = tempo_processor_.Pull(data, static_cast<int>(maxlen))) == 0) {
+      int dev_read = static_cast<int>(ReverseAwareRead(data, maxlen));
+
+      if (!dev_read) {
+        break;
+      }
+
+      tempo_processor_.Push(data, dev_read);
+    }
+
+  } else {
+    // If we aren't doing any tempo processing, simply passthrough the read signal
+    read_count = ReverseAwareRead(data, maxlen);
+  }
+
+  return read_count;
 }
 
 qint64 AudioOutputDeviceProxy::writeData(const char *data, qint64 maxSize)