decoder_api: submit the song tag to the music pipe

When a new song starts playing, send its tag (song->tag) to the music
pipe.  This allows output plugins to render tags for all songs, not
only those with embedded tags understood by the decoder plugin.

src/decoder_internal.h

@@ -30,6 +30,13 @@ struct decoder {
 
 	bool seeking;
 
+	/**
+	 * The tag from the song object.  This is only used for local
+	 * files, because we expect the stream server to send us a new
+	 * tag each time we play it.
+	 */
+	struct tag *song_tag;
+
 	/** the last tag received from the stream */
 	struct tag *stream_tag;