If you went to a rock-concert film in the ’70s, chances are that some of it was going to be in split screen: the trés counterculture technique of dividing the big screen into two parts (or maybe three or four), each one depicting the musical action from a different angle. What made split screen more than a bit trippy was the heady simultaneity of it: It was an invitation to absorb the same event, the same moment , in different ways, which amounted to a kind of stoned cinematic Cubism. (It also anticipated aspects of the digital age.) There were Hollywood dramas that made memorable use of split screen, notably “The Boston Strangler,” and famously “The Thomas Crown Affair” (though I always found the use of it in that film gimmicky). But there’s no doubt that the “Citizen Kane” of split screen was “Woodstock,” where much of the technique was orchestrated by one of the film’s then-unknown editors, Martin Scorsese.…