As Clea DuVall'sHappiest Season— the gay rom-com starring Kristen Stewart and Mackenzie Davis — heads into production in Pittsburgh, the movie has rounded out its supporting cast.Mary Steenburgen, Victor Garber and Schitt's Creekcreator and star Dan Levy are joining the TriStar and Entertainment One project, along withGLOWstar Alison Brie and Aubrey Plaza.Happiest Seasonfollows a woman (Stewart) whose plan to propose to her girlfriend (Davis) while at her family’s annual holiday party is