Fix gallery dismiss transition from sheet-presented VC

Closes #490

Packages/GalleryVC/Sources/GalleryVC/GalleryDismissAnimationController.swift

@@ -52,15 +52,22 @@ class GalleryDismissAnimationController: NSObject, UIViewControllerAnimatedTrans
             appliedSourceToDestTransform = false
         }
         
-        to.view.frame = container.bounds
+        // Moving `to.view` to the container is necessary when the presenting VC (i.e., `to`)
+        // is in the window's root presentation.
+        // But it breaks when the gallery is presented from a sheet-presented VC--in which case
+        // `to.view` is already in the view hierarchy at this point; and adding it to the
+        // container causees it to be removed when the transition completes.
+        if to.view.superview == nil {
+            to.view.frame = container.bounds
+            container.addSubview(to.view)
+        }
+        
         from.view.frame = container.bounds
+        container.addSubview(from.view)
 
         let content = itemViewController.takeContent()
         content.view.translatesAutoresizingMaskIntoConstraints = true
         content.view.layer.masksToBounds = true
-        
-        container.addSubview(to.view)
-        container.addSubview(from.view)
         container.addSubview(content.view)
         
         content.view.frame = destFrameInContainer