"Doug's 1st Movie'' is a thin and less than thrilling feature-length version of a Saturday morning animated series, unseen by me. Chatter on the Web suggests it was originally intended to go straight to video, but was rechanneled into theaters after the startling success of "The Rugrats Movie.'' Since Doug originally started on Nickelodeon, where "Rugrats'' resides, the decision made sense--or would have if this had been a better movie. The plot: Skeeter, the best pal of 12-year-old Doug, finds a polluted pond and is about to take a photo when a nasty trick is played on him by some schoolmates, who pretend to be a monster. Then a real monster emerges from the waters behind him.