Add content rect to Fragment.

Test preservation of lines with boxes.

Test boxes with no text or only a space.

Allow trimmed whitespace to generate empty fragments.

BREAKING: Output may now contain more fragments with zero glyphs,
representing runs of whitespace that was trimmed away. These runs may
also start a box at the end of a line, instead of the beginning of the
next line.

BREAKING: Trimmed whitespace now affects line height.

Test with AlignStart by default.

Test centred paragraph.

Allow paragraphRect without point 0,0.

Test right-aligned paragraph.

Test handling of empty lines.

Test mixed line height.

Improve BiDi handling of numbers and neutrals.

Test with only directionally neutral characters.

Test mirrored neutral characters.

Add tests for mixed text direction.

Describe box edges with extensible type.

Test interaction between boxes and mixed script.

Test line breaking in boxes.

Handle ancestor boxes in rich layout.

Add ancestor boxes to interface.

Test layout with mixed font sizes.