Fix race condition between creating a window and drawing on it

Reason:
Xshm is used to draw on windows (via original Xlib / c extension).
So there are at least two connections to the X11 server.
-> Race condition between drawing on windows (Xlib)
   and creating them (python-Xlib)

ueberzug/ui.py

@@ -231,7 +231,7 @@ class OverlayWindow:
             event_mask=X.StructureNotifyMask)
         self._set_click_through()
         self._set_invisible()
-        self._display.flush()
+        self._display.sync()
 
     def reset_terminal_info(self):
         """Resets the terminal information of this window."""