[ios, osx] Exclude offline cache from backups

Avoid backing up the offline cache onto iCloud, because it can be redownloaded and would easily take up a significant portion of a standard iCloud account. Ideally, we’d even put ambiently cached resources in Caches, so it can be reclaimed by the system when disk space runs low. But unfortunately it has to live in the same file as offline resources.

platform/darwin/src/MGLOfflineStorage.mm

@@ -49,6 +49,12 @@
         NSURL *cacheURL = [cacheDirectoryURL URLByAppendingPathComponent:fileName];
         NSString *cachePath = cacheURL ? cacheURL.path : @"";
         
+        // Avoid backing up the offline cache onto iCloud, because it can be
+        // redownloaded. Ideally, we’d even put the ambient cache in Caches, so
+        // it can be reclaimed by the system when disk space runs low. But
+        // unfortunately it has to live in the same file as offline resources.
+        [cacheURL setResourceValue:@YES forKey:NSURLIsExcludedFromBackupKey error:NULL];
+        
         // Move the offline cache from v3.2.0-beta.1 to a location that can also
         // be used for ambient caching.
         NSString *legacyCacheFileName = @"offline.db";