Improve Header performance a bit (#3556)

* Improve `Header` performance a bit

I have been investigating `<Header />` component performance once again today with `render` cycles in particular. I have observed that during `push` phase, a typical Header in NavigationPlayground re-renders 3 times on iOS and 2 times on Android.

The first time is obvious on both platforms since that's when we add a new scene to an array. Second on iOS was also self-explanatory - we measure title width and other layout parameters in order to provide a better animation.

What got me thinking was the last render cycle that didn't have a clear origin. After digging around I've found it is caused by a `scenes` array changing when transition finishes. What is surprising, it's just the reference to the object that changes, the content (items inside) stay the same.

I have found out that this is caused by `Transitioner` looping through an array of scenes attempting to remove stale scenes. Since scene becomes stale when you `pop` from it, this is obviously a noop when `pushing` a new route. That, obviously, causes an extra render cycle since `filter` produces same, but with a different pointer, object.

* Update Transitioner.js

* Update Transitioner.js

packages/react-navigation/src/views/Transitioner.js

@@ -179,9 +179,19 @@ class Transitioner extends React.Component {
     const prevTransitionProps = this._prevTransitionProps;
     this._prevTransitionProps = null;
 
+    const scenes = this.state.scenes.filter(isSceneNotStale);
+
     const nextState = {
       ...this.state,
-      scenes: this.state.scenes.filter(isSceneNotStale),
+      /**
+       * Array.prototype.filter creates a new instance of an array
+       * even if there were no elements removed. There are cases when
+       * `this.state.scenes` will have no stale scenes (typically when
+       * pushing a new route). As a result, components that rely on this prop
+       * might enter an unnecessary render cycle.
+       */
+      scenes:
+        this.state.scenes.length === scenes.length ? this.state.scenes : scenes,
     };
 
     this._transitionProps = buildTransitionProps(this.props, nextState);