Sundance, which will celebrate its 30th anniversary this year, has always been about making directors feel important. But the independent film festival has also, in recent years, become an increasingly successful marketplace, attracting a cornucopia of celebrity and commercial savoir-faire. What used to be an understated showcase for offbeat independent film in a small, snowy corner of Utah has now turned into a pre-Oscars bidding jamboree between major film companies. Elizabeth Day reports.