rotate method
Implementation
void rotate(VoidCallback callback) {
canvas!.save();
canvas!.translate(center.dx, center.dy);
canvas!.rotate(angle);
callback();
canvas!.restore();
}
void rotate(VoidCallback callback) {
canvas!.save();
canvas!.translate(center.dx, center.dy);
canvas!.rotate(angle);
callback();
canvas!.restore();
}