Don't restore audio device during exporting (#4083)

Fixes deadlock on multi-track export with SDL

include/Mixer.h

@@ -175,6 +175,7 @@ public:
 	void setAudioDevice( AudioDevice * _dev,
 				const struct qualitySettings & _qs,
 							bool _needs_fifo );
+	void storeAudioDevice();
 	void restoreAudioDevice();
 	inline AudioDevice * audioDev()
 	{

include/ProjectRenderer.h

@@ -88,7 +88,6 @@ private:
 
 	AudioFileDevice * m_fileDev;
 	Mixer::qualitySettings m_qualitySettings;
-	Mixer::qualitySettings m_oldQualitySettings;
 
 	volatile int m_progress;
 	volatile bool m_abort;

include/RenderManager.h

@@ -65,6 +65,7 @@ private:
 	void restoreMutedState();
 
 	const Mixer::qualitySettings m_qualitySettings;
+	const Mixer::qualitySettings m_oldQualitySettings;
 	const OutputSettings m_outputSettings;
 	ProjectRenderer::ExportFileFormats m_format;
 	QString m_outputPath;