paint method
Draw itself and its children, according to the calculated
box.offset
Implementation
@override
void paint(Context context) {
super.paint(context);
if (_child != null) {
final mat = Matrix4.identity();
mat.translate(box!.x, box!.y);
context.canvas
..saveContext()
..setTransform(mat);
_child!.paint(context);
context.canvas.restoreContext();
}
}