“Saw,” if you recall, was the story of a nut job named Jigsaw (Tobin Bell) who devised absurdly complex death traps for his victims. His twisted ambition, borrowed from “Se7en,” was to impart moral lessons with extreme force and facile irony. His true raison d’etre, however, was to give two young Australian cineastes a pretext for splattering the images and ideas of every horror movie they’d ever seen – all of them, from the look of it – up on the screen.