Splattery horror movie booths compete with Nickelodeon promoters. Scanty costumes walk the floor along with Snoopy. And there are kids here? How should you navigate?SAN DIEGO — Director Jen Soska and her twin sister came to Comic-Con with one gory aim: Gross out as many people as possible with blood-soaked footage from their upcoming independent horror movie, "American Mary." And indeed, clips from the film about a broke medical student who starts performing underground surgeries attracted a healthy crowd of onlookers to a room in the San Diego convention center.