It’s a rare occasion when someone comes along who ends up being a genuine game changer in his or her particular field of endeavor. But, when such individuals make their presence felt, they leave an indelible mark on their craft, changing it forever. In the field of film criticism, that distinction belongs to Roger Ebert (1942-2013), who almost single-handedly altered the way we look at movies and whose storied life is now the subject of the engaging new documentary, “Life Itself.”