Use SDL_Point instead of rolling our own point struct

The `point` struct was a relic of ActionScript and was added because of
the Flash 'point' object. However, it seems like Simon Roth didn't
realize that SDL has its own point struct.

With this, `Maths.h` can be un-included from a couple headers, which
exposes the fact that `preloader.cpp` was relying on `Maths.h` being
transitively included from `Graphics.h`.

desktop_version/src/Maths.h

@@ -13,10 +13,4 @@ float inline fRandom(void)
     return ( float(rand()) / float(RAND_MAX)) ;
 }
 
-struct point
-{
-    int x;
-    int y;
-};
-
 #endif /* MATHGAME_H */