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/KeyPoll.cpp

@@ -1,5 +1,6 @@
 #include "KeyPoll.h"
 #include "Graphics.h"
+#include "Music.h"
 #include <stdio.h>
 #include <string.h>
 #include <utf8/unchecked.h>
@@ -54,6 +55,8 @@ KeyPoll::KeyPoll()
 	}
 
 	linealreadyemptykludge = false;
+
+	pauseStart = 0;
 }
 
 void KeyPoll::enabletextentry()
@@ -254,6 +257,11 @@ void KeyPoll::Poll()
 					}
 				}
 				SDL_DisableScreenSaver();
+				if (Mix_PlayingMusic())
+				{
+					// Correct songStart for how long we were paused
+					music.songStart += SDL_GetPerformanceCounter() - pauseStart;
+				}
 				break;
 			case SDL_WINDOWEVENT_FOCUS_LOST:
 				isActive = false;
@@ -267,6 +275,7 @@ void KeyPoll::Poll()
 					);
 				}
 				SDL_EnableScreenSaver();
+				pauseStart = SDL_GetPerformanceCounter();
 				break;
 
 			/* Mouse Focus */