add StartDecodeThread to fix potential GetSampleRate race

(will break sound drivers until I update them)

stepmania/src/arch/Sound/RageSoundDriver_Generic_Software.cpp

@@ -288,12 +288,17 @@ void RageSound_Generic_Software::StopMixing( RageSoundBase *snd )
 }
 
 
+void RageSound_Generic_Software::StartDecodeThread()
+{
+	ASSERT( !m_DecodeThread.IsCreated() );
+	m_DecodeThread.Create( DecodeThread_start, this );
+}
+
 RageSound_Generic_Software::RageSound_Generic_Software()
 {
 	shutdown_decode_thread = false;
 
 	m_DecodeThread.SetName("Decode thread");
-	m_DecodeThread.Create( DecodeThread_start, this );
 }
 
 RageSound_Generic_Software::~RageSound_Generic_Software()

stepmania/src/arch/Sound/RageSoundDriver_Generic_Software.h

@@ -86,6 +86,10 @@ class RageSound_Generic_Software: public RageSoundDriver
 	bool GetDataForSound( sound &s );
 
 protected:
+	/* Start the decoding.  This should be called once the hardware is set up and
+	 * GetSampleRate will return the correct value. */
+	void StartDecodeThread();
+	
 	/* Override this to set the priority of the decoding thread, which should be above
 	 * normal priority but not realtime. */
 	virtual void SetupDecodingThread() { }