Change #6684: Cutting point overrides for music base sets

This improves bad looping of title screen song from Windows TTD, and fixes
a long silence at the end of "Can't get there from here" from Windows TTD.

src/base_media_base.h

@@ -301,6 +301,8 @@ struct MusicSongInfo {
 	const char *filename;    ///< file on disk containing song (when used in MusicSet class, this pointer is owned by MD5File object for the file)
 	MusicTrackType filetype; ///< decoder required for song file
 	int cat_index;           ///< entry index in CAT file, for filetype==MTT_MPSMIDI
+	int override_start;      ///< MIDI ticks to skip over in beginning
+	int override_end;        ///< MIDI tick to end the song at (0 if no override)
 };
 
 /** All data of a music set. */