decoder_control: add attributes start_ms, end_ms

Don't read song.start_ms and song.end_ms, let the player thread manage
this logic instead.

src/decoder_thread.c

@@ -369,7 +369,7 @@ decoder_run_song(struct decoder_control *dc,
 {
 	struct decoder decoder = {
 		.dc = dc,
-		.initial_seek_pending = song->start_ms > 0,
+		.initial_seek_pending = dc->start_ms > 0,
 		.initial_seek_running = false,
 	};
 	int ret;