Add a PreCleanPool() static method to pool item types.

(cherry picked from commit 87142ed840)

src/core/pool_func.hpp

@@ -196,6 +196,7 @@ DEFINE_POOL_METHOD(void)::FreeItem(size_t index)
 DEFINE_POOL_METHOD(void)::CleanPool()
 {
 	this->cleaning = true;
+	Titem::PreCleanPool();
 	for (size_t i = 0; i < this->first_unused; i++) {
 		delete this->Get(i); // 'delete NULL;' is very valid
 	}

src/core/pool_type.hpp

@@ -286,6 +286,13 @@ struct Pool : PoolBase {
 		 * @note it's called only when !CleaningPool()
 		 */
 		static inline void PostDestructor(size_t index) { }
+
+		/**
+		 * Dummy function called before a pool is about to be cleaned.
+		 * If you want to use it, override it in PoolItem's subclass.
+		 * @note it's called only when CleaningPool()
+		 */
+		static inline void PreCleanPool() { }
 	};
 
 private: