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/windows/window.rs

@@ -142,6 +142,9 @@ impl Window {
         }
     }
 
+    #[inline]
+    pub fn pre_present_notify(&self) {}
+
     #[inline]
     pub fn outer_position(&self) -> Result<PhysicalPosition<i32>, NotSupportedError> {
         util::WindowArea::Outer.get_rect(self.hwnd())