For such a quietly observant film, unhurried in pace and grounded in daily affairs, Richard Linklater’s Boyhood makes a lot of threats to end in violence. Drinking and driving, horseplay with flying saw blades, inexpert handling of a shotgun, assault with a blunt but shattering object: this is only a partial list of the potentially lethal activities that are made to coalesce, or sometimes erupt, around the central character, a boy named Mason, as he grows up in present-day Texas. Functionally, the notes of menace are useful for ratcheting up the tension now and then in a story that otherwise flows smoothly along Mason’s course, from city to city, school to school, parent to parent. It occurs to me, though, that a thematic purpose also animates these recurring moments of…