LibCards: Make find_card_stack_to_drop_on() non-const

This function returns a card stack that we might want to drop a card on,
so intuitively it should be a mutating function.

Userland/Libraries/LibCards/CardGame.cpp

@@ -59,14 +59,14 @@ ErrorOr<void> CardGame::pick_up_cards_from_stack(Cards::CardStack& stack, Gfx::I
     return {};
 }
 
-RefPtr<CardStack> CardGame::find_stack_to_drop_on(CardStack::MovementRule movement_rule) const
+RefPtr<CardStack> CardGame::find_stack_to_drop_on(CardStack::MovementRule movement_rule)
 {
     auto bounds_to_check = moving_cards_bounds();
 
     RefPtr<CardStack> closest_stack;
     float closest_distance = FLT_MAX;
 
-    for (auto const& stack : stacks()) {
+    for (auto& stack : stacks()) {
         if (stack == moving_cards_source_stack())
             continue;
 

Userland/Libraries/LibCards/CardGame.h

@@ -43,7 +43,7 @@ public:
     Gfx::IntRect moving_cards_bounds() const;
     RefPtr<CardStack> moving_cards_source_stack() const { return m_moving_cards_source_stack; }
     ErrorOr<void> pick_up_cards_from_stack(CardStack&, Gfx::IntPoint click_location, CardStack::MovementRule);
-    RefPtr<CardStack> find_stack_to_drop_on(CardStack::MovementRule) const;
+    RefPtr<CardStack> find_stack_to_drop_on(CardStack::MovementRule);
     ErrorOr<void> drop_cards_on_stack(CardStack&, CardStack::MovementRule);
     void clear_moving_cards();