Certain classical-era painters speak to our current moment more forcefully than others. Sporting a transgressive biography — in which sex and violence loom large — to match a virtuosic, naturalistic style, Michelangelo Merisi da Caravaggio is one such epoch-leaping renegade. The four-hundredth anniversary last year of his mysterious death brings out fresh ruminations from Andrew Graham-Dixon, who casts his scholarly eye on the artist’s headlong life and work.