During the 2010s, Tetsuya Nakashima felt like the closest thing Japan had to its own Nicolas Winding Refn, a purveyor of violent, hyperstylized nihilism that somehow managed to command a mainstream audience. With “The Brightest Sun,” his first film in eight years, the boundary-pushing director of “Confessions” (2010) and “The World of Kanako” (2014) offers his boldest provocation yet: hope.