The haunting documentary “Tower” revisits a 1966 mass shooting at the University of Texas at Austin that shocked the country. It may be difficult to comprehend the reaction to the horror of Charles Whitman, a 25-year-old student who shot more than a dozen dead, wounding more than twice as many. A cover story in Life magazine suggested just how alien the carnage seemed at the time, noting that during the rampage Whitman’s actions were “so outrageous, so hard to grasp, that people could not believe it.” Many more mass shootings later, it’s now tragically easy to believe.