Change OpenTTD#6684: Force theme song on title screen

Theme song is whatever song is in the "theme" slot in the baseset music definition.
If the baseset doesn't define a theme song, this change should keep playing from
the playlist as usual, but that's untested. All known music sets have a theme.

src/base_media_base.h

@@ -308,6 +308,7 @@ struct MusicSet : BaseSet<MusicSet, NUM_SONGS_AVAILABLE, false> {
 	/** The name of the different songs. */
 	MusicSongInfo songinfo[NUM_SONGS_AVAILABLE];
 	byte num_available;
+	bool has_theme;
 
 	bool FillSetDetails(struct IniFile *ini, const char *path, const char *full_filename);
 };

src/music.cpp

@@ -123,6 +123,8 @@ bool MusicSet::FillSetDetails(IniFile *ini, const char *path, const char *full_f
 	bool ret = this->BaseSet<MusicSet, NUM_SONGS_AVAILABLE, false>::FillSetDetails(ini, path, full_filename);
 	if (ret) {
 		this->num_available = 0;
+		this->has_theme = !StrEmpty(this->files[0].filename);
+
 		IniGroup *names = ini->GetGroup("names");
 		IniGroup *catindex = ini->GetGroup("catindex");
 		for (uint i = 0, j = 1; i < lengthof(this->songinfo); i++) {

src/music_gui.cpp

@@ -245,6 +245,16 @@ static void StopMusic()
 
 static void PlayPlaylistSong()
 {
+	if (_game_mode == GM_MENU && BaseMusic::GetUsedSet()->has_theme) {
+		/* force first song (theme) on the main menu.
+		 * this is guaranteed to exist, otherwise
+		 * has_theme would not be set. */
+		_music_wnd_cursong = 1;
+		DoPlaySong();
+		_song_is_active = true;
+		return;
+	}
+
 	if (_cur_playlist[0] == 0) {
 		SelectSongToPlay();
 		/* if there is not songs in the playlist, it may indicate
@@ -270,8 +280,19 @@ void ResetMusic()
 	DoPlaySong();
 }
 
+/**
+ * Check music playback status and start/stop/song-finished
+ * Called from main loop.
+ */
 void MusicLoop()
 {
+	static GameMode _last_game_mode = GM_BOOTSTRAP;
+	bool force_restart = false;
+	if (_game_mode != _last_game_mode) {
+		force_restart = true;
+		_last_game_mode = _game_mode;
+	}
+
 	if (!_settings_client.music.playing && _song_is_active) {
 		StopMusic();
 	} else if (_settings_client.music.playing && !_song_is_active) {
@@ -280,13 +301,13 @@ void MusicLoop()
 
 	if (!_song_is_active) return;
 
-	if (!MusicDriver::GetInstance()->IsSongPlaying()) {
-		if (_game_mode != GM_MENU) {
+	if (force_restart || !MusicDriver::GetInstance()->IsSongPlaying()) {
+		if (_game_mode == GM_MENU && BaseMusic::GetUsedSet()->has_theme) {
+			ResetMusic();
+		} else {
 			StopMusic();
 			SkipToNextSong();
 			PlayPlaylistSong();
-		} else {
-			ResetMusic();
 		}
 	}
 }