Interpolate activity zone prompt fading in and out

To make it real smooth, just in case it was noticeable that it only
updated at 1000/34 FPS before (well, except in slowmode, it's really
noticeable THERE).

Also this removes the re-typing out of (game.act_fade/10.0f) for every
single R, G, and B in gamerender().

desktop_version/src/Logic.cpp

@@ -1516,6 +1516,7 @@ void gamelogic()
     }
 #endif
 
+    game.prev_act_fade = game.act_fade;
     if (game.activeactivity > -1)
     {
         if (game.act_fade < 5)