TTML: Support missing 'extent' tag on <tt>

### Issue description
TtmlDecoder.parseTtsExtent does not support _extent_ defined in pixel such as :
```



```
The region in that case is not considered as :
`Log.w(TAG, "Ignoring non-pixel tts extent: " + ttsExtent);` (line 244)
which leads to :
`Log.w(TAG, "Ignoring region with missing tts:extent: " + regionOrigin);` (line 336)

It looks from the specification [tts:extent](https://www.w3.org/TR/2018/REC-ttml2-20181108/#style-attribute-extent) that "100% 100%" could be a valid answer.

### Version of ExoPlayer being used
I have looked at both 2.11.3 and 2.11.4 version