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.cpp

@@ -150,6 +150,7 @@ void Game::init(void)
     completestop = false;
     activeactivity = -1;
     act_fade = 0;
+    prev_act_fade = 0;
     backgroundtext = false;
     startscript = false;
     newscript = "";