Add Music_LevelVolume, Music_ResetLevelVolume: change music volume from gameplay contexts

- Tunes opt into using this volume by setting
  Tune::use_level_volume to true

src/music.cpp

@@ -31,6 +31,7 @@ void Music_Init(void)
 		tune.fade_out = 1500;
 		tune.fade_out_inclusive = false;
 		tune.resume_fade_in = 750;
+		tune.use_level_volume = true;
 		tune.sync = true;
 		tune.credit = true;
 		tune.vapes = true;
@@ -417,3 +418,13 @@ void Music_BatchExempt(const char* id)
 		tune->resist_once = true;
 	}
 }
+
+void Music_LevelVolume(int volume)
+{
+	g_tunes.level_volume(volume, false);
+}
+
+void Music_ResetLevelVolume(void)
+{
+	g_tunes.level_volume(100, true);
+}

src/music.h

@@ -103,6 +103,14 @@ void Music_Loop(const char *id, boolean loop);
 // as Music_StopAll.
 void Music_BatchExempt(const char *id);
 
+// Set the volume for level context. TODO: this should be
+// done on a more selective basis, rather than globally.
+void Music_LevelVolume(int volume);
+
+// Reset volume back to normal. This will fade it as if the
+// music is resuming after another tune ended.
+void Music_ResetLevelVolume(void);
+
 
 //
 // Query properties.

src/music_manager.cpp

@@ -79,7 +79,13 @@ void TuneManager::tick()
 	{
 		if (load())
 		{
-			I_FadeInPlaySong(tune->resume ? tune->resume_fade_in : tune->fade_in, tune->loop);
+			I_PlaySong(tune->loop);
+			I_FadeSongFromVolume(
+				tune->use_level_volume ? level_volume_ : 100,
+				0,
+				tune->resume ? tune->resume_fade_in : tune->fade_in,
+				nullptr
+			);
 			seek(tune);
 
 			adjust_volume();
@@ -116,6 +122,19 @@ void TuneManager::tick()
 			tune->ending = true;
 		}
 	}
+
+	if (level_volume_ != old_level_volume_ && tune->use_level_volume && tune->can_fade_out)
+	{
+		if (volume_fade_)
+		{
+			I_FadeSong(level_volume_, tune->resume_fade_in, nullptr);
+		}
+		else
+		{
+			I_SetInternalMusicVolume(level_volume_);
+		}
+		old_level_volume_ = level_volume_;
+	}
 }
 
 void TuneManager::pause_unpause() const

src/music_manager.hpp

@@ -88,6 +88,15 @@ public:
 		}
 	}
 
+	void level_volume(int vol, bool fade)
+	{
+		if (vol != old_level_volume_)
+		{
+			level_volume_ = vol;
+			volume_fade_ = fade;
+		}
+	}
+
 private:
 	std::unordered_map<std::string, Tune> map_;
 	std::string current_song_;
@@ -99,6 +108,10 @@ private:
 
 	bool gme_; // hack
 
+	int level_volume_ = 100;
+	int old_level_volume_ = 100;
+	bool volume_fade_ = false;
+
 	decltype(map_)::const_iterator current_iterator() const
 	{
 		return std::max_element(

src/music_tune.hpp

@@ -44,6 +44,9 @@ public:
 	// resumes.
 	int resume_fade_in = 0;
 
+	// Adjust volume based on level context.
+	bool use_level_volume = false;
+
 	// Sync this tune to game logic.
 	bool sync = false;
 

src/p_setup.cpp

@@ -8205,6 +8205,8 @@ void P_LoadLevelMusic(void)
 		tic_t level_music_start = starttime + (TICRATE/2);
 		Music_Seek("level", (std::max(leveltime, level_music_start) - level_music_start) * 1000UL / TICRATE);
 	}
+
+	Music_ResetLevelVolume();
 }
 
 /** Loads a level from a lump or external wad.