Prevent selecting Secret Lab if game isn't at full speed

The game won't let you select the Secret Lab if you're in invincibility
mode, probably so you can't set illegitimate Super Gravitron records
just by standing there and doing nothing.

However, for some reason, it'll still let you select the Secret Lab even
if you've slowed down the game. For consistency, let's prevent selecting
the Secret Lab if the game isn't running at fullspeed, too.

desktop_version/src/Input.cpp

@@ -917,7 +917,7 @@ void menuactionpress()
         }
         else if (game.currentmenuoption == 1 && game.unlock[8])
         {
-            if(!map.invincibility){
+            if(!map.invincibility && game.slowdown == 30){
                 game.mainmenu = 11;
                 graphics.fademode = 2;
             }else{