[fxtf-drafts] Pull Request: [motion] Center numbers in circles

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

== [motion] Center numbers in circles ==
The offset-rotate example image has numbers centered in circles.

We use flexbox to center the numbers, horizontally and vertically.

See https://github.com/w3c/fxtf-drafts/pull/182