Mixer, FifoBuffer, AudioDevice: removed memory allocation/free cycles

There's really no need to allocate a buffer each period, push it to the
FifoBuffer and free it when fetching the buffer in
AudioDevice::getNextBuffer(). Instead keep the pointer in FifoBuffer's
pool and reuse it.

include/fifo_buffer.h

@@ -41,6 +41,10 @@ public:
 		m_size( _size )
 	{
 		m_buffer = new T[_size];
+		for( int i = 0; i < m_size; ++i )
+		{
+			m_buffer[i] = NULL;
+		}
 		m_readerSem.acquire( _size );
 	}
 
@@ -50,9 +54,15 @@ public:
 		m_readerSem.release( m_size );
 	}
 
-	void write( T _element )
+	T nextWriteBuffer()
 	{
 		m_writerSem.acquire();
+		T buf = m_buffer[m_writerIndex];
+		return buf;
+	}
+
+	void write( T _element )
+	{
 		m_buffer[m_writerIndex++] = _element;
 		m_writerIndex %= m_size;
 		m_readerSem.release();