LibCards+Games: Move mark_intersecting_stacks_dirty() to CardGame

As part of this, made a const overload for `Card::rect()`. We need the
non-const one too as it's used for modifying the position of a card
that's being dragged. I plan on changing that soon but we'll see.

Userland/Games/Spider/Game.h

@@ -70,7 +70,6 @@ private:
     void start_timer_if_necessary();
     void update_score(int delta);
     void draw_cards();
-    void mark_intersecting_stacks_dirty(Card& intersecting_card);
     void detect_full_stacks();
     void detect_victory();
     void move_focused_cards(CardStack& stack);