use static mutex member in RtAudioOutput class to serialize RtAudio calls across instances (#32)

include/openauto/Projection/RtAudioOutput.hpp

@@ -50,7 +50,7 @@ private:
     uint32_t sampleRate_;
     SequentialBuffer audioBuffer_;
     std::unique_ptr<RtAudio> dac_;
-    std::mutex mutex_;
+    static std::mutex mutex_;
 };
 
 }

openauto/Projection/RtAudioOutput.cpp

@@ -24,6 +24,8 @@ namespace openauto
 namespace projection
 {
 
+std::mutex RtAudioOutput::mutex_;
+
 RtAudioOutput::RtAudioOutput(uint32_t channelCount, uint32_t sampleSize, uint32_t sampleRate)
     : channelCount_(channelCount)
     , sampleSize_(sampleSize)