LESLIE NIELSEN was halfway through his career before he found his true calling. Hidden beneath his leading-man persona--his handsome visage, athletic carriage, and intelligent demeanor--was a drop-dead funny comedian. Mr. Nielsen, star of "Airplane!" and other laugh-out-loud features, died Sunday at the age of 84.Born in Canada the son of a Mountie, Mr. Nielsen took training as an aerial gunner in World War II but never saw action (too young). After the war, drawn by the allure of acting, he entered the Lorne Greene Academy of Radio Arts in Toronto, and by 1948 was getting parts in the exploding world of TV. Steady work there led to his feature film debut in 1956 in "The Vagabond King."............