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/Game.h

@@ -327,6 +327,7 @@ public:
     bool backgroundtext;
 
     int activeactivity, act_fade;
+    int prev_act_fade;
 
     bool press_left, press_right, press_action, press_map;