TTML: font-size conversion does not support two `<length>`s

[alexandercerutti]

TTML adapter is still in the working, but I can surely create the issues for the features that won't be shipped with the first version of it.

tts:fontSize supports providing two <length>. According to the conversion table on the standard page, TTML

adds optional second length (or percentage) for specifying separate horizontal and vertical scaling of glyph's EM square;

However, to achieve such vertical scaling (perhaps horizontal? The opposite dimention of font-size), we need to change how the renderer works.

Specifically, along with font-size, we need to emit a transform: scaleX()/scaleY(), with the appropriate scaling factor, and apply it on an element that owns display: inline-block (hence, three properties to emit).

This translates, for the renderer, that it should create a new depth-level, a new span or whatever, that will let it specify such styles without impacting the previous or past sections.