Move `Mix_PausedMusic()` call into wrapper function

This wrapper function is for (a) future-proofing (b) proactive
prevention of future copy-pasting (c) to clarify that we never actually
halt music in the SDL_mixer sense, we only pause it, so to check if the
music is halted we actually check if the music is paused instead. This
is important because Mix_PlayingMusic() does not check if the music is
paused and Mix_PausedMusic() does not check if the music is halted.

desktop_version/src/Music.cpp

@@ -428,7 +428,7 @@ void musicclass::processmusic(void)
     }
 
     /* This needs to come after processing fades */
-    if (nicefade && Mix_PausedMusic() == 1)
+    if (nicefade && halted())
     {
         play(nicechange);
         nicechange = -1;
@@ -554,3 +554,8 @@ void musicclass::resumeef(void)
 {
     Mix_Resume(-1);
 }
+
+bool musicclass::halted(void)
+{
+    return Mix_PausedMusic() == 1;
+}

desktop_version/src/Music.h

@@ -44,6 +44,8 @@ public:
     void pauseef(void);
     void resumeef(void);
 
+    bool halted(void);
+
     std::vector<SoundTrack> soundTracks;
     std::vector<MusicTrack> musicTracks;
     SoundSystem soundSystem;