Fix resumemusic/musicfadein not working

It seems like they were unfinished. This commit makes them properly
work.

When a track is stopped with stopmusic() or musicfadeout(),
resumemusic() will resume from where the track stopped. musicfadein()
does the same but does it with a gradual fade instead of suddenly
playing it at full volume.

I changed several interfaces around for this. First, setting currentsong
to -1 when music is stopped is handled in the hook callback that gets
called by SDL_mixer whenever the music stops. Otherwise, it'd be
problematic if currentsong was set to -1 when the song starts fading out
instead of when the song actually ends.

Also, music.play() has a few optional arguments now, to reduce the
copying-and-pasting of music code.

Lastly, we have to roll our own tracker of music length by using
SDL_GetPerformanceCounter(), because there's no way to get the music
position if a song fades out. (We could implicitly keep the music
position if we abruptly stopped the song using Mix_PauseMusic(), and
resume it using Mix_ResumeMusic(), but ignoring the fact that those two
functions are also used on the unfocus-pause (which, as it turns out, is
basically a non-issue because the unfocus-pause can use some other
functions), there's no equivalent for fading out, i.e. there's no
"fade out and pause when it fully fades out" function in SDL_mixer.) And
then we have to account for the unfocus-pause in our manual tracker.

Other than that, these commands are now fully functional.

desktop_version/src/Music.h

@@ -13,11 +13,13 @@ class musicclass
 public:
 	void init();
 
-	void play(int t);
+	void play(int t, const double position_sec = 0.0, const int fadein_ms = 3000);
+	void resume(const int fadein_ms = 0);
 	void haltdasmusik();
 	void silencedasmusik();
 	void fadeMusicVolumeIn(int ms);
 	void fadeout();
+	void fadein();
 	void processmusicfadein();
 	void processmusic();
 	void niceplay(int t);
@@ -53,6 +55,9 @@ public:
 	bool usingmmmmmm;
 
 	binaryBlob musicReadBlob;
+
+	Uint64 songStart;
+	Uint64 songEnd;
 };
 
 extern musicclass music;