Masterpiece isn’t a word to be thrown around lightly, but there’s no denying that Mark Morris’s “L’Allegro, il Penseroso ed il Moderato” is thrillingly that. Choreographed in 1988, when Mr. Morris was the director of dance at the Théâtre Royal de la Monnaie in Brussels, this production, at long last, has been preserved on film by “Great Performances” and can be seen on PBS on Friday night. (Check local listings.)