f7af58951d
The origin may be negative if the subtitle starts off-screen but extends into the screen for example. TTML1 defines `tts:origin` in terms of [`length`](https://www.w3.org/TR/2018/REC-ttml1-20181108/#style-value-length), which allows a prefix of `+`, `-` or nothing, and it's the [same in TTML2](https://www.w3.org/TR/2018/REC-ttml2-20181108/#style-value-length). PiperOrigin-RevId: 682379845