As proud as Independent Film Festival Boston organizers are that their 10th year is underway, they’re prouder yet of the festival’s history of nurturing breakout stars. This year, Newton native Alex Karpovsky is the subject of that festival pride, with a microburst of new projects that includes roles in three films showing at IFFB, as well as a small part in an upcoming Coen brothers’ movie, and suddenly steady work as a cast member on the recently debuted HBO series, “Girls.”