Grim Natwick, the man who gave the world Betty Boop and Snow White in animated form, isn`t saying, but rumor has it that if he had allowed a birthday cake at his party Friday night in the Circle Gallery on North Michigan Avenue, he would have blown out 99 candles.It`s hard to believe after spending a few moments with this animated character, but the paperwork all says-even if he won`t-that he`s one summer shy of a century.It was 59 years ago that Natwick dipped into his inkwell and came up with a vixen who blew kisses, lifted her skirts and batted her eyelashes all over the comic pages for a decade. With the topography of Mae West and a singing style patterned after Boop-Oop-a-Doop Girl Helen Kane, the little vamp also made more than 100 films before her final curtain in 1939. Besides the girls (he also animated Snow White for the Walt Disney Studios), Natwick had a hand in creating quite a strip of cartoon boys-Mickey Mouse, Mr. Magoo, Popeye and Woody Woodpecker among them. With progeny of such standing, it`s no wonder almost 300 friends of the master turned out for his 99th birthday bash. The animator`s efforts to keep his birthday secret were somewhat undermined by the folks at the Circle Gallery, who sent out an avalanche of party invitations-5,000 to be exact. Guests who dropped in off the boulevard at sunset were greeted by a larger-than-life Betty Boop climbing the gallery stairs. The animator, who semiretired in 1968 and now lives in Chillicothe, Mo., basked under the first two limited-edition Boop lithographs, price-tagged at $750 and $850.