[fxtf-drafts] Pull Request: [motion] Define 'sides' value for ray size.

ewilligers has just submitted a new pull request for https://github.com/w3c/fxtf-drafts:

== [motion] Define 'sides' value for ray size. ==
The size argument of ray() is no longer optional.

The new keyword 'sides' indicates an angle-specific path length that
extends to the ray's intersection with the containing box.

Discussed in the CSS Working Group:
https://github.com/w3c/fxtf-drafts/issues/73#issuecomment-296084456

Resolves #73

See https://github.com/w3c/fxtf-drafts/pull/156