A year ago, writer-director Rick Famuyiwa was struggling to find financing for his film after studios in Hollywood passed on it. Now, he’s sitting in a restaurant at Cannes riding the sensation of “Dope,” his comic Los Angeles coming-of-age tale that sparked a bidding war this year at the Sundance Film Festival. “Dope” premiered Friday in the Cannes Film Festival’s Directors’ Fortnight section with most of its young cast enthusiastically in tow. While many indies that make it to Cannes arrive with a small retinue, “Dope” is here in full force — and some cast members even paid their own way to come. “Dope” is a buoyant, stereotype-busting high-school movie about a geek in Inglewood, California, who’s aiming to get into Harvard but is thrust into a rollicking adventure when drug dealers looking for a hiding place stuff his backpack full of Ecstasy.