metal: make post_commit_margin configurable

src/compositor.rs

@@ -507,6 +507,7 @@ fn create_dummy_output(state: &Rc<State>) {
         vblank_event: Default::default(),
         latch_event: Default::default(),
         presentation_event: Default::default(),
+        flip_margin_ns: Default::default(),
     });
     let dummy_workspace = Rc::new(WorkspaceNode {
         id: state.node_ids.next(),