DLangUI: Rendering a generated image

[Alex Parrill via Digitalmars-d-learn]

I found the `ImageWidget` class in controls.d, but I'm still 
having the same issue.

Tweaked code (replaced the section between `window.mainWidget = 
baseLayout;` and `baseLayout.addChild((new Button())...`

	auto img = Ref!DrawBuf(new ColorDrawBuf(W,H));
	draw(cast(ColorDrawBuf) (img.get));
	
	auto drawableImg = Ref!ImageDrawable(new ImageDrawable(img));
	auto canvas = new ImageWidget();
	canvas.drawable = drawableImg;
	baseLayout.addChild(canvas);