handy invalid handle macro

engine/include/sophia/handles.hpp

@@ -24,6 +24,15 @@ struct Handle {
   [[nodiscard]] auto operator<=>(const Handle&) const = default;
 };
 
+struct InvalidHandle {
+  template <typename T>
+  [[nodiscard]] constexpr operator Handle<T>() const {
+    return Handle<T>::invalid();
+  }
+};
+
+inline constexpr InvalidHandle INVALID_HANDLE{};
+
 class Mesh;
 class Panel;
 class RenderTarget;