Feature: Decoder for DOS version music

This is based on reverse-engineering the TTD DOS driver for General MIDI music.

src/base_media_base.h

@@ -285,8 +285,13 @@ static const uint NUM_SONGS_AVAILABLE = 1 + NUM_SONG_CLASSES * NUM_SONGS_CLASS;
 /** Maximum number of songs in the (custom) playlist */
 static const uint NUM_SONGS_PLAYLIST  = 32;
 
+/* Functions to read DOS music CAT files, similar to but not quite the same as sound effect CAT files */
+char *GetMusicCatEntryName(const char *filename, size_t entrynum);
+byte *GetMusicCatEntryData(const char *filename, size_t entrynum, size_t &entrylen);
+
 enum MusicTrackType {
 	MTT_STANDARDMIDI, ///< Standard MIDI file
+	MTT_MPSMIDI,      ///< MPS GM driver MIDI format (contained in a CAT file)
 };
 
 /** Metadata about a music track. */
@@ -295,6 +300,7 @@ struct MusicSongInfo {
 	byte tracknr;            ///< track number of song displayed in UI
 	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
 };
 
 /** All data of a music set. */