message: Use std::move in MeshAssimp.
  Tested this via material_sandbox, seems to be fine.
  Fixes
diff: @@ -305,7 +305,7 @@ MeshAssimp::~MeshAssimp() {
  template<typename T>
  struct State {
  std::vector<T> state;
  - explicit State(std::vector<T>&& state) : state(state) { }
  + explicit State(std::vector<T>&& state) : state(std::move(state)) { }
  static void free(void* buffer, size_t size, void* user) {
  auto* const that = static_cast<State<T>*>(user);
  delete that;
  