The words “spontaneity” and “fluidity” have informed the processes of countless artists. But there’s something else in the art of Ian Cheng, whose video simulations garner lives of their own. His virtual worlds are designed to self-evolve. Characters morph or go off-map. Showings can be engrossing and quick-moving, or they can be like one session of “Emissary in the Squat of Gods” last year, when a virtual girl simply stared at piece of volcanic ash for a couple hours, as viewers looked on.