Remove "valid" flag from Rect and Point, derive them directly from
SDL_Rect and SDL_Point structures, simplify logic. Now x/y/w/h member
variables are directly accessible and Rect/Point may be passed as
SDL_Rect/SDL_Point via pointer or reference.
Change all cases where Null Rects and Points were used to Optional.
invalid state related functions like Null(), IsNull() and Get() are
now deprecated but are not removed yet for compatibility sake.
This is not really needed as SDL has these checks internally,
but this way it's still safer and more apparent that moved-from
objects are handled properly.
