use higher priority queue for image loading

ImageViewer/ViewController.swift

@@ -267,12 +267,14 @@ final class ViewController: UIViewController, UIGestureRecognizerDelegate {
     }
 
     func saveGlobalState() {
-        do {
-            try JSONEncoder().encode(globalState).write(
-                to: documentsURL.appendingPathComponent("state.json")
-            )
-        } catch {
-            print("failed to save global state")
+        queue.async {
+            do {
+                try JSONEncoder().encode(self.globalState).write(
+                    to: self.documentsURL.appendingPathComponent("state.json")
+                )
+            } catch {
+                print("failed to save global state")
+            }
         }
     }
 
@@ -1785,7 +1787,8 @@ class ImageLoader {
         let token = CancellationToken()
         activeTokens[key] = token
 
-        queue.async { [weak self] in
+        // This should be fine even for preloading because of how heavy work is cached and useless work cancelled.
+        queue.async(qos: .userInitiated) { [weak self] in
             guard let self = self else { return }
 
             func finish() {
@@ -1795,6 +1798,7 @@ class ImageLoader {
                     self.loadingTasks.removeValue(forKey: key)
                 }
             }
+
             func setImage(_ image: UIImage) {
                 DispatchQueue.main.async {
                     if let callback = self.loadingTasks[key] {
@@ -1803,8 +1807,6 @@ class ImageLoader {
                 }
             }
 
-            if token.isCancelled { finish(); return }
-
             func extractData() -> Data? {
                 var extractedData = Data()
                 if self.archives.object(forKey: archiveURL.lastPathComponent as NSString) == nil {