Improve the front layer border radius

[danielborges93]

The border radius is not painted properly when the front layer is a colored Container.

Code example

// My theme
final appTheme = ThemeData(
  primaryColor: Color(0xfffafafa), // white
  primaryColorDark: Color(0xff00a651) // green
);

// My BackdropScafold
class MainScreen extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return BackdropScaffold(
      title: Text('Title...'),
      backLayer: Center(
        child: Text('Back'),
      ),
      frontLayer: Container(
        color: Theme.of(context).primaryColorDark,
        child: Center(
          child: Text('Front'),
        ),
      ),
      animationCurve: Curves.easeInOut,
    );
  }
}

Before

After

[WieFel]

If you make the requested change I'll merge it!