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