Factor out slowdown/invincibility conds to function

This factors out the slowdown and invincibility conditionals to a
function. This means less copy-pasted code, and it also conveys intent
(that we don't want to allow competitive options if we have either of
these cheats enabled).

This function isn't implemented in the header because then we would have
to include Map.h for map.invincibility, and transitive includes are
evil. Although, map.invincibility ought to be on Game instead (it was
only mapclass due to 2.2-and-previous argument passing), but that's a
bunch of variable reshuffling that can be done later.

desktop_version/src/Game.h

@@ -451,6 +451,10 @@ public:
         return inintermission || insecretlab || intimetrial || nodeathmode;
     }
 
+    bool incompetitive(void);
+
+    bool nocompetitive(void);
+
     bool over30mode;
     bool glitchrunnermode; // Have fun speedrunners! <3 Misa