Removed alloca calls from sound reader classes because putting it inside a loop tends to cause a stack overflow.

Removed alloca call from png loading because it does not work with setjmp and longjmp.  png loading now uses exceptions instead when an error occurs.

src/RageSoundReader_SpeedChange.cpp

@@ -89,7 +89,7 @@ int RageSoundReader_SpeedChange::FillData( int iMaxFrames )
 		if( iBytesToRead <= 0 )
 			return m_iDataBufferAvailFrames;
 
-		float *pTempBuffer = (float *) alloca( iBytesToRead );
+		float pTempBuffer[iBytesToRead/sizeof(float)];
 		int iGotFrames = m_pSource->Read( pTempBuffer, iFramesToRead );
 		if( iGotFrames < 0 )
 		{