[motion-path] Define offset-rotate at sharp path segment boundaries

[karip]

The Motion Path Level 1 offset-rotate should say what is the rotation at sharp path segment boundaries. I suggest that the direction of the end of the previous segment is used. Google Chrome seems to implement this, as shown by this example: https://jsfiddle.net/e371jukL/1/

Note that the orientation isn’t the average between incoming and outgoing tangents. It would cause annoying flickering in animations, because it would be displayed only for one frame.

[karip]

Sorry! I filed this at wrong repository. It should have been in fxtf-drafts. I've moved this to w3c/fxtf-drafts#108 .