clear cache on memory warning, not just on disappear

deltachat-ios/Controller/GalleryViewController.swift

@@ -97,9 +97,11 @@ class GalleryViewController: UIViewController {
         setupContextMenuIfNeeded()
     }
 
-    override func viewDidDisappear(_ animated: Bool) {
-        // user leaves view and may not come back soon: clearing cache may free a significant amount of memory
-        galleryItemCache = [:]
+    override func didReceiveMemoryWarning() {
+        super.didReceiveMemoryWarning()
+        if !isOnScreen() {
+            galleryItemCache = [:]
+        }
     }
 
     override func viewWillLayoutSubviews() {

deltachat-ios/Extensions/Extensions.swift

@@ -57,6 +57,10 @@ extension UIViewController {
     @objc func dismissKeyboard() {
         view.endEditing(true)
     }
+
+    func isOnScreen() -> Bool {
+        return viewIfLoaded?.window != nil
+    }
 }
 
 extension UIAlertController.Style {