When the same thread repeatedly calls ClipCursor, a message window is
created for every call, then stored in x11drv_thread_data->clip_hwnd.

The WM_X11DRV_CLIP_CURSOR notification is then sent to the desktop
window, then to the previous clipping thread in order for it to destroy
its clip_hwnd. But as the clipping thread is the same, and because
x11drv_thread_data->clip_hwnd has been overwritten, it does not satisfy
the "hwnd == data->clip_hwnd" condition and the window leaked.
Signed-off-by: Rémi Bernon <rbernon@codeweavers.com>
Signed-off-by: Alexandre Julliard <julliard@winehq.org>