shell/screenshot: Plug memory leak [v42]

grab_screenshot_content() does not follow the usual pattern of

  task = g_task_new (...);
  g_task_run_in_thread (task, ...);
  g_object_unref (task);

That means we hold the original reference throughout the operation, and must release it ourselves when done.

#5501 (closed)

(cherry picked from commit 66ae4c7f)