protected override void UpdatePreview(GimpPreview preview)
{
// Fix me: it's probably better to just create a new Drawable iso
// a completely new image!
var clone = new Image(_image);
clone.Crop(preview.Bounds);
var drawable = clone.ActiveDrawable;
var renderer = new Renderer(Variables);
renderer.Render(clone, drawable, true);
preview.Redraw(drawable);
clone.Delete();
}