Fade out music on scenario start and on disabling it

changelog

@@ -1,6 +1,7 @@
 Version 1.13.5+dev:
  * Music and sound effects:
    * Added a preference to pause the music when the game loses focus.
+   * Now the storyscreen music fades out at the start of each scenario.
  * Units:
    * Changed the sound for the melee attack of the
      Loyalist Bowman, Orcish Crossbowman and Orcish Slurbow.

data/core/macros/sound-utils.cfg

@@ -628,6 +628,7 @@
         [music]
             name={MUSIC}
             immediate=yes
+            ms_after=2000
         [/music]
     [/event]
 #enddef
@@ -661,6 +662,7 @@
             name={SCENARIO_MUSIC}
             immediate=yes
             append=no
+            ms_after=2000
         [/music]
     [/event]
 #enddef

players_changelog

@@ -5,15 +5,16 @@ changelog: https://github.com/wesnoth/wesnoth/blob/master/changelog
 Version 1.13.5+dev:
  * Music and sound effects:
    * Added a preference to pause the music when the game loses focus.
+   * Now the storyscreen music fades out at the start of each scenario.
  * Units:
    * Changed the sound for the melee attack of the
 	 Loyalist Bowman, Orcish Crossbowman and Orcish Slurbow.
  * Performance:
-  * When a heuristic determines that it's probably faster, the game predicts battle
-    outcome by simulating a few thousand fights instead of calculating exact
-    probabilities. This method is inexact, but in very complex battles (extremely
-    high HP, drain, slow, berserk, etc.) it's significantly faster than the default
-    damage calculation method.
+   * When a heuristic determines that it's probably faster, the game predicts battle
+     outcome by simulating a few thousand fights instead of calculating exact
+     probabilities. This method is inexact, but in very complex battles (extremely
+     high HP, drain, slow, berserk, etc.) it's significantly faster than the default
+     damage calculation method.
 
 Version 1.13.5:
  * Campaigns:

src/sound.cpp

@@ -61,6 +61,7 @@ unsigned music_refresh_rate = 20;
 bool want_new_music = false;
 int fadingout_time=5000;
 bool no_fading = false;
+bool unload_music = false;
 
 // number of allocated channels,
 const size_t n_of_channels = 32;
@@ -410,12 +411,8 @@ void reset_sound() {
 
 void stop_music() {
 	if(mix_ok) {
-		Mix_HaltMusic();
-
-		std::map<std::string,Mix_Music*>::iterator i;
-		for(i = music_cache.begin(); i != music_cache.end(); ++i)
-			Mix_FreeMusic(i->second);
-		music_cache.clear();
+		Mix_FadeOutMusic(500);
+		Mix_HookMusicFinished([](){ unload_music = true; });
 	}
 }
 
@@ -613,9 +610,22 @@ void music_thinker::process(events::pump_info &info) {
 			if(Mix_PlayingMusic()) {
 				Mix_FadeOutMusic(fadingout_time);
 			}
+
+			unload_music = false;
 			play_new_music();
 		}
 	}
+
+	if (unload_music) {
+		for (auto track : music_cache) {
+			Mix_FreeMusic(track.second);
+		}
+		music_cache.clear();
+
+		Mix_HookMusicFinished(nullptr);
+
+		unload_music = false;
+	}
 }
 
 music_muter::music_muter() :