The sporadic but brilliant career of screenwriter Melissa Mathison offers a vivid case study of an artist who succeeded in maximizing the benefits of each opportunity she was afforded. Though she had fewer than a half dozen films to her credit as of 1996, Mathison earned a permanent place in American pop culture as the writer of the reigning domestic box-office champ, "E.T., the Extra-Terrestrial" (1982). With its 1994 selection for inclusion in the National Film Registry of the Library of Congress, the film was officially certified a classic. Clearly this would have been an enviable credit