Fix !_noFlushOnEnd not flushing (#17212)

This simply copies a bit more from `VtEngine::EndPaint`'s
`_noFlushOnEnd` handling which already seems to fix the linked issue.

Closes #17204

(cherry picked from commit 0b76c51)
Service-Card-Id: 92516193
Service-Version: 1.21

src/renderer/vt/XtermEngine.cpp

@@ -44,6 +44,10 @@ XtermEngine::XtermEngine(_In_ wil::unique_hfile hPipe,
         // have been reset in the EndPaint call. But since that's not going to
         // happen now, we need to reset it here, otherwise we may mistakenly skip
         // the flush on the next frame.
+        if (!_noFlushOnEnd)
+        {
+            _Flush();
+        }
         _noFlushOnEnd = false;
         return hr;
     }