James Cameron, whose technological wizardry helped make Titanic the highest-grossing film in history, is convinced celluloid is on its way out. And to help send it to its grave, the director says he will start production early next year on one of the most ambitious movies ever made: a high-definition, 3-D digital feature. The topic of his planned movie is a secret. Some Cameron watchers speculate that it might be his long-planned epic about the first manned trip to Mars. But Cameron is making no secret of his belief that the movie-going public is now ready for the biggest transformation in the movie business since talkies were invented.