Somebody
Well you can capture to a lower resolution image with this:
You could change the scroll and zoom so what you want to capture covers the screen, and then you can capture a lower res image.
Or it can be modified to capture an area of a certain size.
Edit: It wasn't always working with webgl on. The trick is you need the canvas to redraw for it to work. So to get it to redraw you need something moving, and at least one object spinning would do that trick.