decoder: replaced music_pipe.audioFormat with dc.out_audio_format

.. and rename dc.audioFormat to dc.in_audio_format.  The music pipe
does not need to know the audio format, and its former "audioFormat"
property indicated the format of the most recently added chunk, which
might be confusing when you are reading the oldest chunks.

src/crossfade.c

@@ -21,6 +21,7 @@
 #include "audio.h"
 #include "pcm_utils.h"
 #include "pipe.h"
+#include "audio_format.h"
 
 #include <assert.h>
 #include <string.h>