[css3-ui] propagating the cursor from the root element (issue 27)

As stated in issue 27 of css3-ui (https://wiki.csswg.org/spec/css3-ui#issue-27) we had resolved to mirror for the cursor property the way the background of the root element propagates to the canvas, without also including propagation from HTML’s body element, which was only there for compat reasons in the case of the background property. The actual edit was still pending.

I’ve attached a patch to do this, with prose closing following what’s at http://dev.w3.org/csswg/css-backgrounds/#special-backgrounds

 - Florian