make GetNumChannels pure virtual

not 1 or 2 anymore; RageSound only supports 1 or 2 but intermediate
filters may have more

stepmania/src/RageSound.cpp

@@ -157,6 +157,7 @@ public:
 	int Read( char *buf, int iFrames ) { return 0; }
 	RageSoundReader *Copy() const { return new RageSoundReader_Silence; }
 	int GetSampleRate() const { return 44100; }
+	unsigned GetNumChannels() const { return 1; }
 	bool IsStreamingFromDisk() const { return false; }
 	int GetNextSourceFrame() const { return 0; }
 	float GetStreamToSourceRatio() const { return 1.0f; }

stepmania/src/RageSoundReader.h

@@ -13,7 +13,7 @@ public:
 	virtual ~RageSoundReader() { }
 	virtual RageSoundReader *Copy() const = 0;
 	virtual int GetSampleRate() const = 0;
-	virtual unsigned GetNumChannels() const { return 2; } /* 1 or 2 */
+	virtual unsigned GetNumChannels() const = 0;
 	virtual bool IsStreamingFromDisk() const = 0;
 	virtual bool SetProperty( const RString &sProperty, float fValue ) { return false; }