don't clear damage when the window is not managed

the gstreamer encoder can end up calling 'gstreamer_continue_damage' via a timer, after we've stopped managing the window,
which triggers the 'composite window destroyed outside the UI thread' warning

xpra/x11/models/core.py

@@ -407,6 +407,8 @@ def do_unmanaged(self, wm_exiting) -> None:
     #########################################
 
     def acknowledge_changes(self) -> None:
+        if not self._managed:
+            return
         c = self._composite
         if not c:
             raise RuntimeError("composite window destroyed outside the UI thread?")