Ensured re-rendering the stack uses the latest scene and props

[grahammendick]

Fixes #635

Was using let { current: allScenes } = useRef but this created a stale closure. It meant the NavigationStack didn't get the latest scene and props from its children. For example, conditional navigation, where dynamically configure the stack of scenes, didn't work. Changing to const const allScenes = useRef fixed it.

But noticed another problem where allScenes was a tempo behind because it was updated in useEffect. So it would take an additional render for NavigationStack to have the latest scenes and props. For example, this wouldn't render 'true'.

const [updated, setUpdated] = useState(false);
useEffect(() => {
  setTimeout(() => {
    setUpdated(true);
  });
}, []);
<NavigationHandler stateNavigator={stateNavigator}>
  <NavigationStack>
    <Scene stateKey="example"><Text>{updated}</Text></Scene>
  </NavigationStack>
</NavigationHandler>

Changed allScenes from ref to const and built it during render instead of waiting for useEffect.

Strangely the same code in NavigationMotion didn't have the stale closure. Well, it was stale but by the time renderScene happened it had the latest scenes and props. But updated NavigationMotion to ensure it never has a stale closure.