[CSSWG][css-backgrounds-3] Minor Update to CSS Backgrounds and Borders L3 CR

The CSS WG has published an updated Candidate Recommendation of the
CSS Backgrounds and Borders Module Level 3

     https://www.w3.org/TR/css-backgrounds-3/

This module contains the features of CSS relating to borders and backgrounds.
The main extensions compared to level 2 are borders consisting of images,
boxes with multiple backgrounds, boxes with rounded corners and boxes with
shadows.

This is a minor update to fix a few small errors, primarily in shadow 
serialization and radius geometry. See the full Changes list:
   https://www.w3.org/TR/2020/CR-css-backgrounds-3-20201222/#changes

This publication also brings this module under the protection of Patent Policy 
2020.

Please review the CR, and send any comments to the www-style mailing list,
<www-style@w3.org>, prefixed with [css-background] (as I did on this message)
or (preferably) file them in the GitHub repository at
   https://github.com/w3c/csswg-drafts/issues

For the CSS WG,
~fantasai