Sometimes we have a cue on an active track that starts within the video's
timeline, but ends beyond the end of the video (maybe just ms beyond, because
different browsers calculate a video's duration differently or you get rounding
errors). We would like to see the onexit event being fired for such cues.

This opens door to a larger question: what to do with cues that start before
the video starts, but end within it? These should also have onenter events. It
would be nice to be able to deal with such cues, too, since in this way you can
create an explicit order of cues that should be displayed as the video starts,
e.g. if you want to display an ad from one track before you display a caption
and render it in the remaining space.

[ Duplicated as WHATWG spec bug at https://www.w3.org/Bugs/Public/show_bug.cgi?id=18864 ]