paint method
Draw itself and its children, according to the calculated
box.offset
Implementation
@override
void paint(Context context) {
  super.paint(context);
  context.canvas.saveContext();
  final mat = Matrix4.identity();
  mat.translate(box!.x, box!.y);
  context.canvas.setTransform(mat);
  for (var child
      in children.sublist(_context.firstChild, _context.lastChild)) {
    child.paint(context);
  }
  context.canvas.restoreContext();
}