feat: Implement smooth resizing on X11 with _NET_WM_SYNC_REQUEST

Without smooth resizing, the window will appear to jitter when it is being
resized. This is because X11 completes the resize before the client gets a
chance to draw a new frame.

This is fixed by using the "sync" extension to ensure that the resize of the
X11 window is synchronized with the server.

Closes #2153

src/changelog/unreleased.md

@@ -57,6 +57,7 @@ changelog entry.
   to send specific data to be processed on the main thread.
 - Changed `EventLoopProxy::send_event` to `EventLoopProxy::wake_up`, it now
   only wakes up the loop.
+- On X11, implement smooth resizing through the sync extension API.
 - `ApplicationHandler::create|destroy_surfaces()` was split off from
   `ApplicationHandler::resumed/suspended()`.