"The Grinch" should gobble up more Thanksgiving box office leftovers than any other movie this weekend. "Everything will take a 50-60% drop, particularly the family films," predicts one insider, anticipating a replay of last weekend's chart, since there are no new wide openings. "You've got to figure that 50% drops are in line," another studio executive agrees. "There's no doubt about that. It always happens." Should someone have opened a picture this weekend? "No," replies a distributor, "not unless you had a teenage film like 'Scream.' It's not a good weekend for box office. People are getting set for the holidays and they're drained. They're tired from Thanksgiving and they've got to get going with their Christmas shopping. We did a quarter of a billion dollars over Thanksgiving, so everybody went to the movies last weekend." Universal and Imagine Enter