Currently when creating hidden window and showing it later the content does not get repainted automatically (unless something else triggers the repaint). This adds public `FlutterDesktopViewControllerForceRedraw()` to trigger repaint. The current implementation triggers this via a call to `SendWindowMetrics` but this should eventually be replaced by an embedder API call.
See: https://github.com/flutter/flutter/issues/75319
See: https://github.com/flutter/flutter/issues/69716