support custom of ScaleAndFadeTransformer in SwiperLayout.DEFAULT

[lemoclone]

The only option now are scale both in X and Y, but sometimes scale only in X is also needed.
Currently this can be done by:
transformer: new ScaleAndFadeTransformer2(fade: 0.3,scale: 0.95), scale: null, fade: null,
And set the transform as following:
child = Transform( transform: Matrix4.identity()..scale(0.96, 1.0, 1.0), alignment: Alignment.center, transformHitTests: false, child: item, );