Ann-Marie Corvin A quarter of this year’s Cartoon Forum pitches are derived from existing IP and “The Upside Down River” – a high concept mini-series – is a fine example.Based on the book “La rivière à l’envers” by award-winning French children’s author Jean-Claude Mourlevat, the 10-part fantasy adventure comes from the 10-year-old Paris-based producer-distributor Dandelooo, whose animation credits include the International Emmy-winning “The Treehouse Stories” and France 3 special