It seems only proper that when Laurel met Hardy for the first time, on film, the meeting should have been not face to face but butt to butt. In “The Lucky Dog,” released in 1921, Oliver Hardy, in the role of a thief who has just grabbed a fistful of cash at gunpoint, tucks it into the back pocket of his pants; or, rather, by mistake, into the pants of a complete stranger, played by Stan Laurel, who is bending over and facing the other way. For a moment, they’re together, cheek to cheek, and, as with all good slapstick, the motion is hard to describe but easy to watch. Their personae are not yet fixed (Hardy, though heavy, is far too poised a gent to be a heavy), and…