By Brad Brooks and Barbara Goldberg LUBBOCK, Texas (Reuters) – A historic winter storm has killed at least 21 people, left millions of Texans without power and spun killer tornadoes into the U.S. Southeast on Tuesday. The brutal cold has engulfed vast swaths of the United States, shuttering COVID-19 inoculation centers and hindering vaccine supplies. It is not expected to relent until the weekend. Officials in Texas drew criticism as the state energy grid repeatedly failed, forcing rolling blackouts. Freezing weather stilled giant wind turbines that dot the West Texas landscape, making it impo…