For as long as Kara Walker has occupied the popular imagination — now 47, she became an art star in her late twenties — she has been synonymous with cut-out silhouettes enacting scenes of the antebellum South. But it took a recent fellowship in Rome for the artist to to see America as it is now — to absorb the hateful rhetoric of the Trump era, to observe the BLM movement boiling over. Her current exhibition of new work at the Cleveland Museum of Art, “The Ecstasy of St. Kara,” a series of forceful, oversized charcoal drawings, is an explosion of scale and detail that feels new to the artist. The violence of the slavery scenes — a black woman is chained by the neck and dragged by a white man — seem to echo the graphic depictions of martyrdom that Caravaggio placed all around Rome. St. Kara, indeed.