song/DetachedSong: copy the AudioFormat from LightSong

Enables the "Format" row in "playlistinfo" responses.

https://github.com/MusicPlayerDaemon/MPD/issues/1094

src/song/DetachedSong.cxx

@@ -28,7 +28,8 @@ DetachedSong::DetachedSong(const LightSong &other) noexcept
 	 tag(other.tag),
 	 mtime(other.mtime),
 	 start_time(other.start_time),
-	 end_time(other.end_time) {}
+	 end_time(other.end_time),
+	 audio_format(other.audio_format) {}
 
 DetachedSong::operator LightSong() const noexcept
 {

src/song/DetachedSong.hxx

@@ -21,6 +21,7 @@
 #define MPD_DETACHED_SONG_HXX
 
 #include "tag/Tag.hxx"
+#include "pcm/AudioFormat.hxx"
 #include "Chrono.hxx"
 #include "util/Compiler.h"
 
@@ -79,6 +80,12 @@ class DetachedSong {
 	 */
 	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();
+
 public:
 	explicit DetachedSong(const char *_uri) noexcept
 		:uri(_uri) {}
@@ -231,6 +238,14 @@ public:
 	gcc_pure
 	SignedSongTime GetDuration() const noexcept;
 
+	const AudioFormat &GetAudioFormat() const noexcept {
+		return audio_format;
+	}
+
+	void SetAudioFormat(const AudioFormat &src) noexcept {
+		audio_format = src;
+	}
+
 	/**
 	 * Update the #tag and #mtime.
 	 *