New York should be grateful that the Yale Center for British Art in New Haven is closed for renovations. As a result, eight canvases by the inimitable English painter George Stubbs, one of the great artists of the 18th century, have been lent to the Metropolitan Museum of Art. Works by Stubbs are scarce in this town: The Met has one painting, and there’s a drawing at the Frick Collection. This makes “Paintings by George Stubbs From the Yale Center for British Art” a rare and thrilling treat.