All roads lead to Italy this season, and not only because the Venice Biennale, the greatest art exhibition of them all, opens there in May. Over in New York, the Metropolitan Museum of Art is staging a Raphael retrospective—the first ever devoted to the Renaissance master in the US, shockingly. In Paris, at the Louvre, another titan of the Renaissance takes center stage: Michelangelo, whose sculptures will be shown alongside Rodin’s. In Vienna, the Kunsthistorisches Museum is giving a big show to Canaletto and his nephew, Bernardo Bellotto. But back to the Biennale. That exhibition is the most high-profile biennial in the world and the star of a year that marks an astonishing convergence of biennials taking place the world over. New York alone is getting two this spring: the Whitney Biennial and Greater New York, at the Whitney Museum and MoMA PS1, respectively. Then, in Pittsburgh, there’s the Carnegie International, and farther afield, in Australia, there’s the Biennale of Sydney.…