private void Workspace_CanvasSizeChanged(object sender, EventArgs e)
{
Requisition req = new Requisition ();
req.Height = PintaCore.Workspace.CanvasSize.Y;
req.Width = PintaCore.Workspace.CanvasSize.X;
drawingarea1.Requisition = req;
drawingarea1.QueueResize ();
}