Navigation bar elements flickering

**Describe the bug**

Some navigation bar elements(`UIBarButtonItem`s and `UINavigationitem`s with custom views) flicker when navigation bar gets hidden interactively.


**To Reproduce**

The issues manifests quite easily, for example when setting up an item with custom image view:
```swift
let imageView = UIImageView(image: UIImage(named: "AppIcon"))
navigationItem.rightBarButtonItem = UIBarButtonItem(customView: imageView)
```
or event just an empty view
```swift
let titleView = UIView(frame: CGRect(x: 0, y: 0, width: 200, height: 30))
titleView.backgroundColor = .white
navigationItem.titleView = titleView
```
Here's a sample project that demonstrates this:
[Sample.zip](https://github.com/andreamazz/AMScrollingNavbar/files/3451411/Sample.zip)

**Expected behavior**
Would be nice if it wouldn't flicker.