Fix major incorrect usage of mrb class definitions causing segfaults

include/utils.h

@@ -152,38 +152,71 @@ struct Rect {
   }
 };
 
-template <typename T>
+template <typename T = void *>
 struct Pool {
-  T &operator[](int index) {
-    return items[index];
-  }
-
-  const std::vector<T> &all() {
-    return items;
-  }
-
-  int acquire(const T &item) {
-    if (!free_indices.empty()) {
-      int index = free_indices.back();
-      free_indices.pop_back();
-      items[index] = std::move(item);
-      return index;
-    } else {
-      items.push_back(std::move(item));
-      return items.size() - 1;
+  T *operator[](int index) {
+    if (!is_valid(index)) {
+      fprintf(stderr, "Invalid pool index: %d\n", index);
+      return nullptr;
     }
+    return slots[index].ptr;
+  }
+
+  const std::vector<T *> &all() {
+    static std::vector<T *> all_items;
+    all_items.clear();
+    for (const auto &slot : slots)
+      if (slot.alive)
+        all_items.push_back(slot.ptr);
+    return all_items;
+  }
+
+  int acquire(T *item) {
+    int index;
+    if (!free_indices.empty()) {
+      index = free_indices.back();
+      free_indices.pop_back();
+    } else {
+      index = slots.size();
+      slots.push_back({});
+    }
+    slots[index].ptr = item;
+    slots[index].refcount = 1;
+    slots[index].alive = true;
+    return index;
+  }
+
+  void use(int index) {
+    if (is_valid(index))
+      slots[index].refcount++;
   }
 
   void release(int index) {
-    free_indices.push_back(index);
+    if (!is_valid(index))
+      return;
+    auto &slot = slots[index];
+    slot.refcount--;
+    if (slot.refcount <= 0) {
+      delete slot.ptr;
+      slot.ptr = nullptr;
+      slot.alive = false;
+      free_indices.push_back(index);
+    }
   }
 
-  bool is_valid(size_t index) const {
-    return index >= 0 && index < items.size() && std::find(free_indices.begin(), free_indices.end(), index) == free_indices.end();
+  bool is_valid(int index) const {
+    return index >= 0
+           && index < (int)slots.size()
+           && slots[index].alive;
   }
 
 private:
-  std::vector<T> items;
+  struct Slot {
+    T *ptr = nullptr;
+    int refcount = 0;
+    bool alive = false;
+  };
+  std::vector<Slot> slots;
   std::vector<int> free_indices;
 };