Fix no-draw frames when exiting a level with custom assets

When exiting a level, music.init() gets called again, and every time it
gets called after the first time it gets called, it will free all music
tracks.

To do so, it calls Mix_FreeMusic(). Unfortunately, if there is music
fading, Mix_FreeMusic() will call SDL_Delay(), which will result in
annoying no-draw frames. Meaning, the screen freezes and doesn't draw
anything, and has to wait a bit before continuing.

Here's the relevant piece of code from SDL2_mixer's music.c:

    if (music == music_playing) {
        /* Wait for any fade out to finish */
        while (music->fading == MIX_FADING_OUT) {
            Mix_UnlockAudio();
            SDL_Delay(100);
            Mix_LockAudio();
        }
        if (music == music_playing) {
            music_internal_halt();
        }
    }

This is especially annoying if you're a TASer, because no-draw frames in
a libTAS movie aren't guaranteed to last for a consistent number of
frames when you change your inputs around.

After this patch, as long as your computer can unmount and re-mount
assets fast enough (it doesn't seem like mine can, unfortunately), then
you won't get any freezes when exiting a level that has custom assets.
(This freeze didn't happen when loading a level because the title screen
music fadeout upon pressing ACTION had enough time to fully complete
before the level got loaded.)

desktop_version/src/Music.cpp

@@ -33,11 +33,18 @@ musicclass::musicclass()
 
 void musicclass::init()
 {
-	for (size_t i = 0; i < soundTracks.size(); ++i) {
+	for (size_t i = 0; i < soundTracks.size(); ++i)
+	{
 		Mix_FreeChunk(soundTracks[i].sound);
 	}
 	soundTracks.clear();
-	for (size_t i = 0; i < musicTracks.size(); ++i) {
+
+	// Before we free all the music: stop playing music, else SDL2_mixer
+	// will call SDL_Delay() if we are fading, resulting in no-draw frames
+	Mix_HaltMusic();
+
+	for (size_t i = 0; i < musicTracks.size(); ++i)
+	{
 		Mix_FreeMusic(musicTracks[i].m_music);
 	}
 	musicTracks.clear();