By Ann Hornaday, The Washington Post AUSTIN, Texas — Every film festival develops its own bubble — a terrarium-like alternate reality that invisibly takes hold between movies, audiences, mood, food and weather. South by Southwest (SXSW), the music-film-technology conference that wraps up this weekend, has come into its own as an invaluable buzz-builder for particular kinds of films, which themselves fit right into the festival’s of-the-moment milieu. That gestalt could be felt throughout SXSW during its opening weekend, when both the film and interactive conferences got underway.