implemented audio exporting

The encoder was moved to its own thread and will transcode the PCM from the
audio renderer into the chosen codec while the video frames are still
received. The implementation isn't perfect and could use some cleaning up, but
it is functional at the moment.

app/core.cpp

@@ -341,6 +341,8 @@ void Core::DeclareTypesForQt()
   qRegisterMetaType<OpenGLTexturePtr>();
   qRegisterMetaType<OpenGLTextureCache::ReferencePtr>();
   qRegisterMetaType<NodeValueTable>();
+  qRegisterMetaType<FramePtr>();
+  qRegisterMetaType<AudioRenderingParams>();
 }
 
 void Core::StartGUI(bool full_screen)