allow changing of music volume through WML,

...slightly modified from patch #1640 by lfernando

changelog

@@ -47,6 +47,7 @@ Version 1.9.0-svn:
    * Add [replace_schedule] tag, which replaces the time of day schedule
    * Trying to include a missing macro/file is now a fatal error
    * Extended #ifdef so that it also tests for files and directories
+   * Add [music_volume] tag, which permits modifications to music volume during scenario
  * Miscellaneous and bug fixes:
    * Defaulted log level to warning again
    * New option to auto-set delay shroud update on game start

src/game_events.cpp

@@ -711,6 +711,25 @@ WML_HANDLER_FUNCTION(music, /*event_info*/, cfg)
 	sound::play_music_config(cfg.get_parsed_config());
 }
 
+WML_HANDLER_FUNCTION(music_volume, /*event_info*/, cfg)
+{
+	std::string absolute = cfg["value"];
+	std::string relative = cfg["relative"];
+	int vol = sound::get_music_volume();
+	
+	if(!absolute.empty()) {
+		vol = atoi(absolute.c_str());
+	}
+
+	if(!relative.empty()) {
+		const float rel = atof(relative.c_str());
+		vol = static_cast<int>(rel*vol/100.0);
+	}
+
+	sound::set_music_volume(vol);
+}
+
+
 WML_HANDLER_FUNCTION(sound, /*event_info*/, cfg)
 {
 	play_controller *controller = resources::controller;

src/sound.cpp

@@ -808,6 +808,10 @@ void play_UI_sound(const std::string& files)
 	}
 }
 
+int get_music_volume()
+{
+	return Mix_VolumeMusic(-1);
+}
 
 void set_music_volume(int vol)
 {

src/sound.hpp

@@ -87,6 +87,7 @@ class music_thinker : public events::pump_monitor {
 // Save music playlist for snapshot
 void write_music_play_list(config& snapshot);
 
+int get_music_volume();
 void set_music_volume(int vol);
 void set_sound_volume(int vol);
 void set_bell_volume(int vol);