Make RTL text overflow the left paragraph edge.

This behaviour is consistent with CSS Text Module Level 3.

BREAKING: Previously, all text would overflow the right paragraph edge,
regardless of text direction.

Fix handling of mixed line height.

Displace lines by the height of the tallest fragment in the previous
line, not the shortest one.

Implement unions of lists of rectangles.

Parametrise rectangle union bias.

Remove TODO which would require an API break.

Simplify code since run direction is always known.

Add TODO for end-of-line whitespace.

Improve BiDi handling of numbers and neutrals.

Autodetect text direction for legacy interface.

Propagate BiDi levels to layout algorithm.

BREAKING: Text direction of the root box is now correctly used for
ordering fragments. Previously, the order was always LTR.

Integrate BiDi levels with spanToRuns algorithm.

Span/box boundaries no longer reset directionality.

BREAKING: Text with no strongly directional characters would
previously be treated as having no direction. It will now be
treated as having the direction defined in TextOptions.
This affects calculated positions.

Convert legacy span directions into levels.

Move BiDi Level to separate module.

Prepare for extending ProtoRun.

Move ScriptCode to appropriate module.

Fix documentation: internal references.

Fix documentation: link to layoutRich.

Fix documentation: missing boxSpacing.

Describe box edges with extensible type.

Expose piecewise paragraph construction.