AudioStream: Make speed changes less poppy

src/util/audio_stream.h

@@ -67,6 +67,7 @@ public:
   /// Nominal rate is used for both resampling and timestretching, input samples are assumed to be this amount faster
   /// than the sample rate.
   void SetNominalRate(float tempo);
+  void UpdateTargetTempo(float tempo);
 
   void SetStretchMode(AudioStretchMode mode);