db/simple: add an AudioFormat to each Song

src/db/LightSong.hxx

@@ -21,6 +21,7 @@
 #define MPD_LIGHT_SONG_HXX
 
 #include "Chrono.hxx"
+#include "AudioFormat.hxx"
 #include "Compiler.h"
 
 #include <string>
@@ -78,6 +79,12 @@ struct LightSong {
 	 */
 	SongTime end_time = SongTime::zero();
 
+	/**
+	 * The audio format of the song, if given by the decoder
+	 * plugin.  May be undefined if unknown.
+	 */
+	AudioFormat audio_format = AudioFormat::Undefined();
+
 	LightSong(const char *_uri, const Tag &_tag) noexcept
 		:uri(_uri), tag(_tag) {}