Interpolate credits position

So that it's as smooth as possible, especially when holding down ACTION
to make it go really fast.

desktop_version/src/Game.cpp

@@ -174,6 +174,7 @@ void Game::init(void)
     activity_g = 0;
     activity_b = 0;
     creditposition = 0;
+    oldcreditposition = 0;
     bestgamedeaths = -1;
 
     fullScreenEffect_badSignal = false;