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/linux/wayland/window/mod.rs

@@ -293,6 +293,11 @@ impl Window {
         self.event_loop_awakener.ping();
     }
 
+    #[inline]
+    pub fn pre_present_notify(&self) {
+        // TODO
+    }
+
     #[inline]
     pub fn outer_size(&self) -> PhysicalSize<u32> {
         let window_state = self.window_state.lock().unwrap();