metal: make post_commit_margin configurable

src/tasks/connector.rs

@@ -179,6 +179,7 @@ impl ConnectorHandler {
             latch_event: Default::default(),
             vblank_event: Default::default(),
             presentation_event: Default::default(),
+            flip_margin_ns: Default::default(),
         });
         on.update_visible();
         on.update_rects();