Cannes’ Un Certain Regard section got off to an outrageous start with the world premiere of “Camp Miasma,” a queer slasher movie that’s equal parts uproariously funny and confoundingly heady. Hannah Einbinder and Gillian Anderson star in the writer-director’s third feature film, which received a six-minute standing ovation on Wednesday night as Schoenbrun hugged their stars to roaring applause and cheers from the crowd. “OK, what should we do now?” Einbinder asked as the applause reached its peak and began to die down. Schoenbrun then said they were speechless as Einbinder thanked those who stuck around until the end. Audience members who got on the movie’s wavelength were audibly vibing with the narrative, while a majority of others were quick to leave the theater right as the screen cut to black. Those who stayed in attendance — including James Franco and Jordan Firstman — championed Schoenbrun and their leading ladies.…