fix: Get context size from DOMRenderer root element size

[alexanderGugel]

#411

Still not sure if we actually want this or if we should just assume that the selected element is "clean", since e.g. the DOMRenderer root element could also be styled via CSS etc.

I don't think it's a big issue (if any).

[michaelobriena]

Need to think about this.

[jd-carroll]

@michaelobriena if the size is retrieved from the root element, I think the nested div should be removed. (IMO I think this should be preferred.) Then it should become part of the API that users should not add other content to the root element.

[alexanderGugel]

@michaelobriena Close or merge?