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();
}