Lowercase the "apply WebVTT cue settings" definition

For consistency with e.g. "collect WebVTT cue timings and settings"
Refactor how and when "apply WebVTT cue settings" is invoked

This factors out the common "Let nodes be" and most importantly makes
the algorithm actually return the CSS boxes.
Merge pull request #82 from foolip/reorganize-rendering

Refactor how and when "apply WebVTT cue settings" is invoked