private void pictureBox1_MouseMove(object sender, MouseEventArgs e)
        {
            if (pnt == nullPt)
            {
                return;
            }
            switch (mode)
            {
                case 0:
                    if (e.Button == MouseButtons.Left)
                    {
                        Graphics g = Graphics.FromImage(pictureBox1.Image);
                        g.DrawLine(pen, pnt, e.Location);
                        g.Dispose();
                        pnt = e.Location;
                        pictureBox1.Invalidate();
                    }
                    break;
                case 1:
                case 2:
                    {
                        if (e.Button == MouseButtons.Left)
                        {
                            ReversibleDraw();
                            movePt = e.Location;
                            ReversibleDraw();

                        } break;
                    }
            }
        }