Until about two decades ago hafu (meaning “half-Japanese”) were viewed with a mixture of awe, envy, a bit of suspicion and a barrage of other emotions, some positive and some not. Now, mixed-race people are no longer that rare, and Japan has become more open about the whole thing. But have things really improved? Megumi Nishikura and Lara Perez Takagi’s documentary “Hafu” takes the bull by the horns and the results are intriguing.