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/orbital/window.rs

@@ -166,6 +166,9 @@ impl Window {
         }
     }
 
+    #[inline]
+    pub fn pre_present_notify(&self) {}
+
     #[inline]
     pub fn reset_dead_keys(&self) {
         // TODO?