Handle Photo Library privacy settings correctly

If photo library access is denied by the user then FreeOTP will no longer attempt to
read from the photo gallery, falling back to the default FreeOTP token icon.

Resolves: #258

FreeOTP/ImageDownloader.swift

@@ -32,6 +32,19 @@ class ImageDownloader : NSObject {
         super.init()
     }
 
+    func isPHAssetAuthorized(_ status: PHAuthorizationStatus) -> Bool! {
+        switch status {
+        case .denied, .restricted:
+            return false
+        case .notDetermined:
+            return nil
+        case .authorized, .limited:
+            return true
+        @unknown default:
+            return false
+        }
+    }
+
     func fromPHAsset(_ asset: PHAsset, completion: @escaping (UIImage) -> Void) {
         let opts: PHImageRequestOptions = PHImageRequestOptions()
         opts.isSynchronous = true
@@ -49,13 +62,28 @@ class ImageDownloader : NSObject {
     }
 
     func fromALAsset(_ asset: URL, completion: @escaping (UIImage) -> Void) {
-        if asset.scheme == "assets-library" {
-            let rslt = PHAsset.fetchAssets(withALAssetURLs: [asset], options: nil)
-            if rslt.count > 0 {
-                return fromPHAsset(rslt[0] , completion: completion)
+        let status = PHPhotoLibrary.authorizationStatus()
+        let authorized = isPHAssetAuthorized(status)
+        var access_granted = false
+
+        if (authorized == false) {
+            // shortcut completion
+        } else if (authorized == nil) {
+            PHPhotoLibrary.requestAuthorization { (status) -> Void in
+                if (self.isPHAssetAuthorized(status) == true) {
+                    access_granted = true
+                }
             }
         }
 
+        if (authorized == true || access_granted == true) {
+            if asset.scheme == "assets-library" {
+                let rslt = PHAsset.fetchAssets(withALAssetURLs: [asset], options: nil)
+                if rslt.count > 0 {
+                    return fromPHAsset(rslt[0] , completion: completion)
+                }
+            }
+        }
         return completion(DEFAULT)
     }