After climbing the run-down steps up to the Church of Sant’Agostino, I often find myself fishing for coins to give to the one or two homeless people who are usually sitting outside, and looking up expectantly with begging cups in front of them. And that’s fitting, given that it is a pair of Romans from the street who take centre stage in this moving and classically counter-intuitive painting by Caravaggio, which is his own riff on the Adoration of the Magi.