Add platform::startup_notify for Wayland/X11

The utils in this module should help the users to activate the windows
they create, as well as manage activation tokens environment variables.

The API is essential for Wayland in the first place, since some
compositors may decide initial focus of the window based on whether
the activation token was during the window creation.

Fixes #2279.

Co-authored-by: John Nunley <jtnunley01@gmail.com>

src/platform_impl/linux/x11/xdisplay.rs

@@ -2,7 +2,10 @@ use std::{
     collections::HashMap,
     error::Error,
     fmt, ptr,
-    sync::{Arc, Mutex},
+    sync::{
+        atomic::{AtomicU32, Ordering},
+        Arc, Mutex,
+    },
 };
 
 use crate::window::CursorIcon;
@@ -32,6 +35,9 @@ pub(crate) struct XConnection {
     /// The index of the default screen.
     default_screen: usize,
 
+    /// The last timestamp received by this connection.
+    timestamp: AtomicU32,
+
     pub latest_error: Mutex<Option<XError>>,
     pub cursor_cache: Mutex<HashMap<Option<CursorIcon>, ffi::Cursor>>,
 }
@@ -95,6 +101,7 @@ impl XConnection {
             xcb: Some(xcb),
             atoms: Box::new(atoms),
             default_screen,
+            timestamp: AtomicU32::new(0),
             latest_error: Mutex::new(None),
             cursor_cache: Default::default(),
         })
@@ -136,6 +143,36 @@ impl XConnection {
     pub fn default_root(&self) -> &xproto::Screen {
         &self.xcb_connection().setup().roots[self.default_screen]
     }
+
+    /// Get the latest timestamp.
+    #[inline]
+    pub fn timestamp(&self) -> u32 {
+        self.timestamp.load(Ordering::Relaxed)
+    }
+
+    /// Set the last witnessed timestamp.
+    #[inline]
+    pub fn set_timestamp(&self, timestamp: u32) {
+        // Store the timestamp in the slot if it's greater than the last one.
+        let mut last_timestamp = self.timestamp.load(Ordering::Relaxed);
+        loop {
+            let wrapping_sub = |a: xproto::Timestamp, b: xproto::Timestamp| (a as i32) - (b as i32);
+
+            if wrapping_sub(timestamp, last_timestamp) <= 0 {
+                break;
+            }
+
+            match self.timestamp.compare_exchange(
+                last_timestamp,
+                timestamp,
+                Ordering::Relaxed,
+                Ordering::Relaxed,
+            ) {
+                Ok(_) => break,
+                Err(x) => last_timestamp = x,
+            }
+        }
+    }
 }
 
 impl fmt::Debug for XConnection {