in progress requests (or portions of a request), but does nothing
 if the request is already complete.

 <p>Unlike <a href="../../../../com/bumptech/glide/request/Request.html#clear--"><code>Request.clear()</code></a>, this method allows implementations to act differently on subparts
 of a request. For example if a Request has both a thumbnail and a primary request and the
 thumbnail portion of the request is complete, this method allows only the primary portion of
 the request to be paused without clearing