config: allow disabling explicit-sync

src/compositor.rs

@@ -224,6 +224,7 @@ fn start_compositor2(
         create_default_seat: Cell::new(true),
         subsurface_ids: Default::default(),
         wait_for_sync_obj: Rc::new(WaitForSyncObj::new(&ring, &engine)),
+        explicit_sync_enabled: Cell::new(true),
     });
     state.tracker.register(ClientId::from_raw(0));
     create_dummy_output(&state);