auto easing = params.size() > 4 ? params.at(4) : L"linear";
user_speed_ = speed_tweener(user_speed_.fetch(), destination_user_speed, frames, tweener(easing));
auto easing = params.size() > 4 ? params.at(4) : L"linear";
user_speed_ = speed_tweener(user_speed_.fetch(), destination_user_speed, frames, tweener(easing));