Fix regression with playing the same track after it fades out

With commit 48313169b6 (PR #453),
AllyTally added a single-case patch for a regression, instead of fixing
it at its root cause.

In fact, that commit only fixes the music if Presenting VVVVVV is
playing while exiting to the menu, not if you enter a level that plays
Presenting VVVVVV - so it only fixes it going one way, and not going the
other way around; neither fixing also all the other cases this could
happen.

It doesn't, say, fix the case where you are exited to the menu
automatically after collecting the last crewmate in the level (or if the
level calls gamestate 1013 itself), which is what happens in my MIRA-VIU
TAS video at the end, and which I noted in the description of that video
( https://www.youtube.com/watch?v=OYQO4ePbYW4&t=111 ).

So, the problem here is that when musicclass::play() is called, it sees
that currentsong is the same as its input, and decides that since the
music is already playing, it shouldn't play the music again. Thus, the
music fades out, and we get silence instead of the music playing again.

But I said this was a regression. Why didn't this happen in 2.2? Well,
it's because of the fact that 2.2 sets currentsong to -1 (no music
playing at all) immediately when starting a fadeout, and not when the
fadeout completes (commit facb079b35,
PR #316). As you can imagine, this discrepancy could lead to bugs, given
that the game would think that music wasn't playing when in actuality it
was, but fixing this bug could also break code that expected this wrong
behavior. And in this case, it has.

So to properly fix the root cause of this, instead of naïvely
single-case patching out every case that comes up randomly, in
musicclass::play(), the function will now ignore if the input given is
the same as currentsong if the music is currently fading out.

desktop_version/src/Music.cpp

@@ -188,7 +188,7 @@ void musicclass::play(int t, const double position_sec /*= 0.0*/, const int fade
 	safeToProcessMusic = true;
 	musicVolume = MIX_MAX_VOLUME;
 
-	if (currentsong == t)
+	if (currentsong == t && Mix_PlayingMusic() != MIX_FADING_OUT)
 	{
 		return;
 	}