winex11: Flush the display after releasing the capture to make sure it happens right away.

dlls/winex11.drv/window.c

diff --git a/dlls/winex11.drv/window.c b/dlls/winex11.drv/window.c
index 99156b6..31800f7 100644
--- a/dlls/winex11.drv/window.c
+++ b/dlls/winex11.drv/window.c
@@ -1958,6 +1958,7 @@
         wine_tsx11_lock();
         XFlush( gdi_display );
         XUngrabPointer( thread_data->display, CurrentTime );
+        XFlush( thread_data->display );
         wine_tsx11_unlock();
         thread_data->grab_window = None;
     }