It's interesting WebVTT explicitly handles line & position differently
in horizontal-rl/lr and vertical-lr/rl contexts. position is always
measured from the left of the viewport, even for rtl text, but line in
vertical-rl is measured from the right of the viewport.
We don't have to make Cue match WebVTT (I can go change the WebVTT
decoding instead) but it seems a reasonable 'default' to follow.
PiperOrigin-RevId: 304353900
