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/Editor.cpp

@@ -610,7 +610,7 @@ void editorrender(void)
     // Draw entities backward to remain accurate with ingame
     for (int i = customentities.size() - 1; i >= 0; i--)
     {
-        point tpoint;
+        SDL_Point tpoint;
         SDL_Rect drawRect;
 
         //if() on screen
@@ -928,7 +928,7 @@ void editorrender(void)
             if (i <= ed.currentghosts) { // We don't want all of them to show up at once :)
                 if (ed.ghosts[i].rx != ed.levx || ed.ghosts[i].ry != ed.levy)
                     continue;
-                point tpoint;
+                SDL_Point tpoint;
                 tpoint.x = ed.ghosts[i].x;
                 tpoint.y = ed.ghosts[i].y;
                 SDL_Color ct = ed.ghosts[i].realcol;