Reps with People for the Ethical Treatment of Animals, or PETA, tell us they will debut a video featuring Cambridge-bred actor Casey Affleck at his alma mater, Cambridge Rindge and Latin School, on Tuesday. Affleck, who voiced a character in the summer movie “ParaNorman,” narrates the PETA short about the dehorning of cows, and urges his fans to support the animals by purchasing nondairy products.