[fxtf-drafts] Pull Request: [motion] Clarify meaning of closest-side

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

== [motion] Clarify meaning of closest-side ==
closest-side and farthest-side are perpendicular distances
measured between the initial position and the relevant sides.

When closest-side or farthest-side are used, and the initial
position is outside the box, the sides are considered to extend
indefinitely.

(When diagonal distances to corners are required, closest-corner
and farthest-corner can be used.)

See https://github.com/w3c/fxtf-drafts/pull/227