Music: Enforce the quick fade time as soon as we know it's happening.

Not every music path will trip the quick_fade bool that resets the timer to
500ms, so we need to do this as soon as it's asked of us. This fixes the fade
when quitting to the main menu.

Fixes #764

desktop_version/src/Music.cpp

@@ -351,7 +351,7 @@ void musicclass::fadeMusicVolumeOut(const int fadeout_ms)
 
 void musicclass::fadeout(const bool quick_fade_ /*= true*/)
 {
-	fadeMusicVolumeOut(2000);
+	fadeMusicVolumeOut(quick_fade_ ? 500 : 2000);
 	quick_fade = quick_fade_;
 }