decoder_api: added function decoder_replay_gain()

This function replaces the replay_gain_info parameter for
decoder_data().  This allows the decoder to announce replay gain
changes, instead of having to pass the same object over and over.

src/decoder_internal.h

@@ -52,6 +52,8 @@ struct decoder {
 
 	/** the chunk currently being written to */
 	struct music_chunk *chunk;
+
+	struct replay_gain_info *replay_gain;
 };
 
 /**