Disney was desperate to find a way to make “Snow White and the Seven Dwarfs,”the first feature-length animated movie ever made, actually feel like a feature-length movie. He’d been doing his best to ignore the naysayers who christened the very idea of the film as “Disney’s folly.” But they did trigger a nagging concern for the 35-year-old studio chief: Audiences might reject an animated movie if it remained stuck in the realm of the flat, two-dimensional shorts that had propelled Mickey Mouse to worldwide celebrity.