If you've been longing to see the worst family entertainment of 1966, A Dog of Flanders may be the movie for you: It's as stilted and icky-wholesome as a bad episode from ''The Wonderful World of Disney.'' In the 18th century, a blond Dutch-boy orphan, living with his grandfather and faithful pooch, dreams of becoming a painter in the spirit of Rubens. Based on a popular children's book, the movie itself is a study in dankly lit farm-country interiors, ham actors uglified by muttonchops, and general quaint tedium. As the kindly artist-aristocrat who becomes the boy's mentor, Jon Voight takes a nothing role and gives it the benefit of his sweetness, a quality that has been all but doused in this once-great actor's recent performances.