x11: Windows are Sync again (#474)

* x11: Windows are Sync again

Fixes #472

* Add test ensuring that Window is Sync

Window must be Sync for Vulkano's Arc<FramebufferAbstract> to be usable.

tests/sync_object.rs

@@ -0,0 +1,9 @@
+extern crate winit;
+
+fn needs_sync<T:Sync>() {}
+
+#[test]
+fn window_sync() {
+    // ensures that `winit::Window` implements `Sync`
+    needs_sync::<winit::Window>();
+}