DecoderAPI: _replay_gain() returns void

Let the function decoder_replay_gain() update
decoder_control::replay_gain_db instead of letting each decoder plugin
take care for that.

src/decoder_api.h

@@ -152,9 +152,8 @@ decoder_tag(struct decoder *decoder, struct input_stream *is,
  * @param decoder the decoder object
  * @param rgi the replay_gain_info object; may be NULL to invalidate
  * the previous replay gain values
- * @return the replay gain adjustment used
  */
-float
+void
 decoder_replay_gain(struct decoder *decoder,
 		    const struct replay_gain_info *replay_gain_info);
 
@@ -162,12 +161,11 @@ decoder_replay_gain(struct decoder *decoder,
  * Store MixRamp tags.
  *
  * @param decoder the decoder object
- * @param replay_gain_db the ReplayGain adjustment used for this song
  * @param mixramp_start the mixramp_start tag; may be NULL to invalidate
  * @param mixramp_end the mixramp_end tag; may be NULL to invalidate
  */
 void
-decoder_mixramp(struct decoder *decoder, float replay_gain_db,
+decoder_mixramp(struct decoder *decoder,
 		char *mixramp_start, char *mixramp_end);
 
 #endif