Almost any of the 16 Giorgio de Chirico paintings in “Horses: The Death of a Rider” could sustain an exhibition by itself. A couple from the late 1920s are less polished, and you could reasonably call “Two Horses on a Seashore,” 1970, a little glib. But for the most part the lush, peculiar and consistently delightful paintings show the Greek-born Italian painter at the top of his game for the better part of five decades.