Make WindowBuilder Send + Sync

Window builder is always accessed by winit on the thread event loop
is on, thus it's safe to mark the data it gets as `Send + Sync`.
Each unsafe object is marked individually as `Send + Sync` instead
of just implementing `Send` and `Sync` for the whole builder.

src/platform_impl/ios/view.rs

@@ -473,7 +473,7 @@ impl WinitUIWindow {
 
         this.setRootViewController(Some(view_controller));
 
-        match window_attributes.fullscreen.clone().map(Into::into) {
+        match window_attributes.fullscreen.0.clone().map(Into::into) {
             Some(Fullscreen::Exclusive(ref video_mode)) => {
                 let monitor = video_mode.monitor();
                 let screen = monitor.ui_screen(mtm);