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();
}