Two specters hover above the Telluride Film Festival as it celebrates its 50th anniversary, starting Aug. 31. Make that three. Since last year’s installment, two of the festival’s quartet of visionary co-founders have died: Bill Pence, 82, and Tom Luddy, 79. Since SAG-AFTRA went on strike in July, questions have dogged the fall film festival circuit about whether actors would be able, or willing, to promote films from the studios represented by the Alliance of Motion Picture and Television Producers. Colman Domingo portrays Civil Rights maverick Bayard Rustin in “Rustin.” At left is Aml Ameen as Martin Luther King.