The signature of the LayoutBuilder builder function.
Must not return null.
The return widget is placed as one of Stack's children.
If change FijkView between normal mode and full screen mode, the panel would
data can be used to pass value from different panel.
typedef FijkPanelWidgetBuilder = Widget Function(FijkPlayer player, FijkData data, BuildContext context, Size viewSize, Rect texturePos);