public ViewportForm(int w, int h, Viewport vp)
 {
     InitializeComponent();
     bmp = new Bitmap(w, h);
     g = Graphics.FromImage(bmp);
     this.vp = vp;
     Width = w;
     Height = h;
 }
 internal void SetViewport(Viewport vp)
 {
     this.vp = vp;
 }