Once inserted, as if by cold metal probe to the cortex, some images can’t be erased. It has taken me eight months and three viewings to wrap my mind around “Possessor.” From the beginning, I recognized Brandon Cronenberg’s brilliant sci-fi puzzle to be more than just another bracingly extreme psychological thriller, but I wasn’t sure what to make of the violence. “Possessor” is by far the most disturbing film I’ve ever seen at Sundance, for reasons that, if I were to list them, would not only spoil the experience but send a certain contingent of shock-horror fans rushing to see what all the fuss was about.