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/Input.cpp

@@ -3195,7 +3195,7 @@ void teleporterinput(void)
         {
             for (size_t i = 0; i < map.teleporters.size(); i++)
             {
-                point& tele = map.teleporters[i];
+                SDL_Point& tele = map.teleporters[i];
 
                 if (map.isexplored(tele.x, tele.y))
                 {