Test line splitting with many runs.

Add ignored prefixes and suffixes to test data.

This ensures that implementation does not rely on the texts having zero
offset inside their byte arrays.

Use ICU to find line break boundaries.

Implement line breaking on code point boundaries.

Use top edge for placing lines.

Implement absolute line heights with half-leadings.

Calculate "normal" line height and pen position.

Upgrade Fragment to contain pen position.

Implement "plain" interface.

Create "plain" interface.

WIP version with WIP interface.