Codechange: Pass a MusicSongInfo struct instead of bare filename to music drivers.

Preparation for later extending the info passed to music drivers.

src/music/music_driver.hpp

@@ -14,6 +14,8 @@
 
 #include "../driver.h"
 
+struct MusicSongInfo;
+
 /** Driver for all music playback. */
 class MusicDriver : public Driver {
 public:
@@ -21,7 +23,7 @@ public:
 	 * Play a particular song.
 	 * @param filename The name of file with the song to play.
 	 */
-	virtual void PlaySong(const char *filename) = 0;
+	virtual void PlaySong(const MusicSongInfo &song) = 0;
 
 	/**
 	 * Stop playing the current song.