ROLLO ROSS "Happiest Season," starring Kristen Stewart, breaks the mold as a romantic comedy about a lesbian couple set around the holidays. Stewart, who is bisexual, says she's astounded it's taken this long for a major studio to get behind such a film. "I would have loved to have grown up with a movie like this. But the fact that I'm 30 years old and I'm in it - I feel really lucky to be in the first film, but I'm also baffled that it is," Stewart said. Stewart plays Abby, who's been in a relationship with Harper (Mackenzie Davis)…