title: when navigationBar.isTranslucent is true, the backgroundImage of navigationbar become translucent too
body: <p>In my first page, the navigation bar should be transparent, and when push to next page, the navigation bar should be a image. I worked it out by changing the <code>_UIBarBackground</code> or <code>_UINavigationBarBackground</code>'s alpha. </p>
  
  <p>But I meet a big problem, in the first page, view should be placed and draw from (0,0) so I set the navigation bar's <code>isTranslucent</code> to <code>true</code>, that all works fine. But when I enter the next page, background image shows with a translucent looking which I don't want.</p>
  
  <p>btw, I set background using: </p>
  
  <pre><code>navVC.navigationBar.setBackgroundImage(UIImage(named: "navigation_bar_background"),
                                         for: .default)
  </code></pre>
  
  <p><a href="https://i.stack.imgur.com/Jhdra.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/Jhdra.png" alt="enter image description here"></a></p>
  
  <p>I checked the image used here, and it's not translucent.</p>
  
  <p>What can I do with this issue? I don't want set viewController's <code>extendedLayoutIncludesOpaqueBars</code> to true due to ugly appearance when animation. </p>