seekToSubtitleLine
Seeks to the start of a subtitle line and returns it once the line's first frame is on screen, with seek's contract (#491). offset 0 is the line showing, or the last line before now when none shows, so a learner who missed a line hears it again; -1 and +1 are the previous and the next line. Lines are the selected subtitle track's cues as far as the player has read them, every cue of an external file and those an embedded track has delivered, at the times they show with the subtitle delay. Repeated calls move one line each, a paused player included, because each counts from where the last one went.
Return
where the line starts, in the item's time.
Throws
when no subtitle track is selected, or there is no such line, for example past the last one read so far, besides seek's own refusals.