The Tribeca Film Festival opens today, but the partying began Tuesday night, when booze flowed freely at a directors party at the Bowery Hotel, in a space that some partygoers said looked like an S&M dungeon. Waitresses in little black dresses with plunging necklines added to the quirky theme. Fans hoping festival co-founder Robert DeNiro would pop in had to be content with a video of him at last year’s festival that was shown over and over and over on large monitors. Now, bring on the movies.