By Alan Zilberman, Special To The Washington Post “Desierto” is a stark, economical thriller. Its conflict is elemental and the characters are defined with bold strokes. Director Jonás Cuarón, the son of acclaimed filmmaker Alfonso Cuarón, draws from the primal motives of “Gravity.” (He wrote that screenplay with his father.) Instead of science fiction, however, this earthbound tale has a poignant political message — and not a subtle one. A truck passes through the desert.