[css-omview] add a offset Property to the getBoxQuads API

[jogibear9988]

The getBoxQuads calculates the transformed corner points of an element, where it is really drawn.
if you want to use these information to draw for example an selection rectangle wich is offset of the element it would be a nice addition to the getBoxQuads API to add an offset property to the options object.
It should be of type DOMQuad, so you could specify an offset for each of the 4 corner points.

I already added this to my polyfill: https://github.com/jogibear9988/getBoxQuadsPolyfill