DreamWorks Animation tries a new tack, embracing sincerity over satire, with “How to Train Your Dragon,” a thrilling drama interspersed with amusing comedic elements (rather than the other way around) from “Lilo and Stitch” directors Chris Sanders and Dean DeBlois. Though inspired by Cressida Cowell’s eight-book kidlit series, “Dragon” is more prequel than adaptation, establishing the Viking-inhabited Isle of Berk, where the brawny warrior clan is cursed with an unruly reptile problem. It falls to misfit Hiccup to rethink their relationship with the species — from pests to pets — a notion so appealing, “Dragon” seems destined to become another cornerstone franchise.