It was the world’s most famous movie built around a slide show: Al Gore, in a darkened auditorium, clicking through images that warned of a heating planet. Within the film’s first minutes, Mr. Gore described a “moral imperative to make big changes,” a call he echoed several times onstage. Twenty years after the release of “An Inconvenient Truth,” his Oscar-winning documentary, it is remembered mostly for its cultural resonance — for bringing awareness of global warming into the mainstream. Mr.