What are we to make of the fact that in both of his final films Heath Ledger played a clown? There was the Joker, of course, in the year’s biggest hit, “The Dark Knight,” and his turn in “The Imaginarium of Dr. Parnassus,” due out next year, in which he plays a member of a magician’s troupe. In the picture’s first released photo, shown here, Ledger is in full brooding “Pagliacci” mode. Tears of a clown? He who laughs last laughs best? Or Ledger’s realization that the heavier the mask, the more freedom for the actor?