Happily N'Ever After is to Pixar what color-by-numbers is to traditional Disney animation. The press notes boast that the total production time was only 15 months, ''unheard of for a first-class computer animated movie.'' Maybe there's a reason why that's unheard-of. This thrown-together, dumpy-looking misfire finds Cinderella's wicked stepma (Sigourney Weaver) taking over Fairy Tale Land. Sounds subversive, but Happily is stuffed with stock characters the vain prince, the critter sidekicks who adamantly stay stock.