Support TTML extent & origin on <p> & <div> tags

[icbaker]

Use case description

TTML2 allows origin and extent to be declared on <p> and <div> tags:
https://www.w3.org/TR/2018/REC-ttml2-20181108/#style-attribute-extent-special-usage

Currently TtmlDecoder only parses extent & origin from region tags (TTML 1 only allowed this):

ExoPlayer/library/core/src/main/java/com/google/android/exoplayer2/text/ttml/TtmlDecoder.java

Line 322 in fd24df3

String regionOrigin = XmlPullParserUtil.getAttributeValue(xmlParser, TtmlNode.ATTR_TTS_ORIGIN);

Proposed solution

Update TtmlDecoder#parseNode to consider tts:origin and tts:extent attributes.

[nigelmegitt]

Worth noting that usage of this feature (TTML2's #region-implied-animation feature) is not permitted in either the IMSC or EBU-TT-D profiles, when considering prioritisation of this vs other features that have a bigger impact for the user such as #linePadding - I know this is a bit cheeky because there's no issue open for supporting linePadding, but in general it would be worth prioritising features that are in the IMSC profiles over those that aren't, IMO.

[ojw28]

Ignore the commit referenced above; the commit referenced the wrong issue.