Add Window::on_present_notify to ack about drawing

That's a way to communicate to winit that you'll present to the window.
While it's a no-op for now, it'll be used to throttle drawing.

src/platform_impl/ios/window.rs

@@ -74,6 +74,8 @@ impl Inner {
         }
     }
 
+    pub fn pre_present_notify(&self) {}
+
     pub fn inner_position(&self) -> Result<PhysicalPosition<i32>, NotSupportedError> {
         unsafe {
             let safe_area = self.safe_area_screen_space();