Move returning to lab to separate variables

Again, what I've done here is removed the over-reliance on Terry's State
Machine to return to the lab, and just moved it into separate variables
instead. This means that returning to the lab is ALMOST entirely
self-contained in MAPMODE, except there's a quick jaunt over to GAMEMODE
to run a script because you can only run scripts in GAMEMODE.

desktop_version/src/Game.h

@@ -383,6 +383,8 @@ public:
     void returntolab();
     bool fadetomenu;
     int fadetomenudelay;
+    bool fadetolab;
+    int fadetolabdelay;
 };
 
 extern Game game;