Defer: Allow empty initialization & add `SetFunc()` method

lib/base/defer.hpp

@@ -22,6 +22,8 @@ public:
 	{
 	}
 
+	Defer() = default;
+
 	Defer(const Defer&) = delete;
 	Defer(Defer&&) = delete;
 	Defer& operator=(const Defer&) = delete;
@@ -39,6 +41,11 @@ public:
 		}
 	}
 
+	inline void SetFunc(std::function<void()> func)
+	{
+		m_Func = std::move(func);
+	}
+
 	inline
 	void Cancel()
 	{