"Slacker" is one of the early-'90s indie watersheds, a smart, shaggy, go-with-the-flow film that follows denizens of Austin, Tex., as they walk, talk and hang out over the course of one day. The roots of Linklater's life-and-how-to-live-it style, perfected in the "Before Sunrise" trilogy as well as "Boyhood," are visible here.