“Degas: A Passion for Perfection” is a blockbuster show in spite of itself, a must-see that does its homework and serves up plenty for others. It’s exhaustive and, for those who indulge in the 100-plus works on the walls at the Denver Art Museum, exhausting. Though not in the way the masses might expect. The exhibit is not fully loaded with those beguiling portraits of ballet dancers that Degas fanatics know and love.