Fix gifv playback not continuing when returning from background

Closes #506

Tusker/Views/Attachments/GifvPlayerView.swift

@@ -22,6 +22,7 @@ class GifvPlayerView: UIView {
 
     let controller: GifvController
     private var presentationSizeCancellable: AnyCancellable?
+    private var wasPlayingWhenSceneBackgrounded = false
     
     override var intrinsicContentSize: CGSize {
         controller.item.presentationSize
@@ -45,4 +46,30 @@ class GifvPlayerView: UIView {
         fatalError("init(coder:) has not been implemented")
     }
     
+    override func willMove(toWindow newWindow: UIWindow?) {
+        super.willMove(toWindow: newWindow)
+        
+        if let oldWindow = window,
+           let oldScene = oldWindow.windowScene {
+            NotificationCenter.default.removeObserver(self, name: UIScene.didEnterBackgroundNotification, object: oldScene)
+            NotificationCenter.default.removeObserver(self, name: UIScene.willEnterForegroundNotification, object: oldScene)
+        }
+        
+        if let newWindow,
+           let newScene = newWindow.windowScene {
+            NotificationCenter.default.addObserver(self, selector: #selector(sceneDidEnterBackground), name: UIScene.didEnterBackgroundNotification, object: newScene)
+            NotificationCenter.default.addObserver(self, selector: #selector(sceneWillEnterForeground), name: UIScene.willEnterForegroundNotification, object: newScene)
+        }
+    }
+    
+    @objc private func sceneDidEnterBackground() {
+        wasPlayingWhenSceneBackgrounded = controller.player.rate > 0
+    }
+    
+    @objc private func sceneWillEnterForeground() {
+        if wasPlayingWhenSceneBackgrounded {
+            controller.play()
+        }
+    }
+    
 }