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/preloader.cpp

@@ -5,6 +5,7 @@
 #include "GraphicsUtil.h"
 #include "KeyPoll.h"
 #include "Localization.h"
+#include "Maths.h"
 #include "UtilityClass.h"
 #include "VFormat.h"