There’s not much to boxing. It’s a sport where two people agree to punch each other until time runs out or one of them is rendered unconscious. Such a blank canvas allows writers and those in the performing arts to add their own flourishes. That’s why there are so many boxing movies, but also why they all end up seeming the same. “Southpaw” (2015) is the latest to glorify the down-on-his-luck pugilist fighting for honor and family. Director Antoine Fuqua and star Jake Gyllenhaal provide modern edginess, but it’s still the same old story, told well, but the same nonetheless.