pipe: added music_buffer, rewrite music_pipe

Turn the music_pipe into a simple music_chunk queue.  The music_chunk
allocation code is moved to music_buffer, and is now managed with a
linked list instead of a ring buffer.  Two separate music_pipe objects
are used by the decoder for the "current" and the "next" song, which
greatly simplifies the cross-fading code.

src/chunk.h

@@ -35,6 +35,9 @@ struct audio_format;
  * music_pipe_append() caller.
  */
 struct music_chunk {
+	/** the next chunk in a linked list */
+	struct music_chunk *next;
+
 	/** number of bytes stored in this chunk */
 	uint16_t length;