In writer-director Mary Bronstein’s “If I Had Legs I’d Kick You,” out Oct. 10 from A24, Christian Slater is more heard than seen. His raspy voice — familiar from a host of memorable movies, from “Heathers” to “Blink Twice,” and a Golden Globe-winning role on “Mr. Robot” — is on the other end of the phone as Rose Byrne begins to depart from reality. Byrne plays a mother whose precarious housing situation and family health crisis push her to the edge; Slater is her husband, away on business and —