[css-compositing] Move CSS Compositing spec to WD

All,

in last week's SVG face-to-face meeting, the CSS compositing and blending
specification was accepted as a Working Draft.
We've asked that the CSS WG do the same and that it is put on this week's
agenda for the weekly conference call.

You can find the latest version here:
https://dvcs.w3.org/hg/FXTF/rawfile/tip/compositing/index.html
Please let us know if you have feedback on the current spec.

Thanks!
  Rik Cabanier
  Adobe Systems, Inc