Correct the fade transition when content modes differ

Previously runCrossDissolveWithContentMode was adding the transitionView to the imageView and settting the imageViews alpha to 0. This resulted in the image immediately disappearing since alpha of subviews are multiplied by the alpha of parent views.

Sources/NukeExtensions/ImageViewExtensions.swift

@@ -400,7 +400,7 @@ extension ImageViewController {
         )
     }
 
-    /// Performs cross-dissolve animation alonside transition to a new content
+    /// Performs cross-dissolve animation alongside transition to a new content
     /// mode. This isn't natively supported feature and it requires a second
     /// image view. There might be better ways to implement it.
     private func runCrossDissolveWithContentMode(imageView: UIImageView, image: ImageContainer, params: ImageLoadingOptions.Transition.Parameters) {
@@ -410,8 +410,13 @@ extension ImageViewController {
         // Create a transition view which mimics current view's contents.
         transitionView.image = imageView.image
         transitionView.contentMode = imageView.contentMode
-        imageView.addSubview(transitionView)
-        transitionView.frame = imageView.bounds
+        if let superview = imageView.superview {
+            superview.insertSubview(transitionView, aboveSubview: imageView)
+            transitionView.frame = imageView.frame
+        } else {
+            imageView.addSubview(transitionView)
+            transitionView.frame = imageView.bounds
+        }
 
         // "Manual" cross-fade.
         transitionView.alpha = 1