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/mod.rs

@@ -555,12 +555,7 @@ impl Window {
         }
     }
     pub fn request_user_attention(&self, request_type: Option<UserAttentionType>) {
-        match self {
-            #[cfg(x11_platform)]
-            Window::X(ref w) => w.request_user_attention(request_type),
-            #[cfg(wayland_platform)]
-            Window::Wayland(ref w) => w.request_user_attention(request_type),
-        }
+        x11_or_wayland!(match self; Window(w) => w.request_user_attention(request_type))
     }
 
     #[inline]
@@ -568,6 +563,11 @@ impl Window {
         x11_or_wayland!(match self; Window(w) => w.request_redraw())
     }
 
+    #[inline]
+    pub fn pre_present_notify(&self) {
+        x11_or_wayland!(match self; Window(w) => w.pre_present_notify())
+    }
+
     #[inline]
     pub fn current_monitor(&self) -> Option<MonitorHandle> {
         match self {

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();

src/platform_impl/linux/x11/window.rs

@@ -1731,6 +1731,11 @@ impl UnownedWindow {
             .unwrap();
     }
 
+    #[inline]
+    pub fn pre_present_notify(&self) {
+        // TODO timer
+    }
+
     #[inline]
     pub fn raw_window_handle(&self) -> RawWindowHandle {
         let mut window_handle = XlibWindowHandle::empty();