Instead of linebreaking inside each single-font text segment, linebreak
the entire inline formatting context at once. This has several benefits:
1. It allows us to use `icu_segmenter` (already in use from style),
which is written against a newer version of the Unicode spec --
preventing breaking emoji clusters.
2. Opens up the possibility of changing the way that linebreaking and
shaping work -- eventually allowing shaping across inline box
boundaries and line breaking *after* shaping.
Co-authored-by: Rakhi Sharma <atbrakhi@igalia.com>
