Option to not force image style creation

Hi,

I'm using this on a client website and the feed was getting really slow to load (>8 seconds). After some digging I found that the culprit was these lines in views_rss-media.find.inc:

if (!file_exists(drupal_realpath($path))) {
  image_style_create_derivative($image_style, $item_uri, $path);
}

Eventually I think it's related to the width/height attributes defined below but since they are not required maybe we could give the option to turn this on/off. Once I commented these lines the load speed dropped below 1 second. Note that it may due to the fact that i'm using the s3fs module and the file_exists seems to return false even when the file is already created.