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.

examples/window.rs

@@ -33,6 +33,8 @@ fn main() -> Result<(), impl std::error::Error> {
                 window.request_redraw();
             }
             Event::RedrawRequested(_) => {
+                // Notify the windowing system that we'll be presenting to the window.
+                window.pre_present_notify();
                 fill::fill_window(&window);
             }
             _ => (),