Move the update UI with metadata call to the mainqueue to avoid crashes when the UI was updated from the background.

This also resolves the only Audio but no Video issue.

Sources/VLCPlaybackController.m

@@ -938,7 +938,7 @@ VLCMediaDelegate>
 {
     if (_needsMetadataUpdate == NO) {
         _needsMetadataUpdate = YES;
-        dispatch_after(dispatch_time(DISPATCH_TIME_NOW, (int64_t)(0.5 * NSEC_PER_SEC)), DISPATCH_QUEUE_PRIORITY_DEFAULT, ^{
+        dispatch_after(dispatch_time(DISPATCH_TIME_NOW, (int64_t)(0.5 * NSEC_PER_SEC)), dispatch_get_main_queue(), ^{
             [self _updateDisplayedMetadata];
         });
     }