public IObservable<IBitmap> GetImage(Uri uri, TimeSpan offset)
     => _cache.LoadImageFromUrl(uri.ToString(), false, null, null, offset.GetAbsoluteUtc());
 public IObservable<IBitmap> GetImage(Uri uri, TimeSpan offset, DesiredImageSize desiredDimensions) {
     var url = uri.ToString();
     return _cache.LoadImageFromUrl(GetDimensionKey(uri, desiredDimensions), url, false, desiredDimensions.Width,
         desiredDimensions.Height,
         offset.GetAbsoluteUtc());
 }