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_thread.c

@@ -31,6 +31,7 @@
 #include "mapper.h"
 #include "path.h"
 #include "uri.h"
+#include "replay_gain.h"
 
 #include <glib.h>
 
@@ -302,6 +303,7 @@ decoder_run_song(struct decoder_control *dc,
 {
 	struct decoder decoder = {
 		.dc = dc,
+		.replay_gain = NULL,
 	};
 	int ret;
 
@@ -329,6 +331,9 @@ decoder_run_song(struct decoder_control *dc,
 	pcm_convert_deinit(&decoder.conv_state);
 
 	/* flush the last chunk */
+	if (decoder.replay_gain != NULL)
+		replay_gain_info_free(decoder.replay_gain);
+
 	if (decoder.chunk != NULL)
 		decoder_flush_chunk(&decoder);