''Empty pools and f---ed up houses'' that have succumbed to the real estate bust in Orange County, Calif., are the natural habitat of low-hopes skateboard rat Josh "Skreech" Sandoval, the subject of Tristan Patterson's outstanding documentary. There's nothing to emulate about such a banged-up 23-year-old ''pool skater.'' Yet with its propulsive punk-rock soundtrack and beautifully rough cinematography, Dragonslayer makes you care about this scrawny young man, skating to nowhere. A-

