Implement vertical alignment.

BREAKING: Boxes now generate struts (CSS), which can make a line taller
than is necessary to contain its fragments.

Do not pass around unnecessary data.

Separate vertical alignment from horizontal positioning.

Store vertical offsets inside ProtoFragment.

Intended to make these offsets manipulable during layout.

Add internal structure for vertical offsets.

Add content rect to Fragment.

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.

Add extensible filter for invisible lines.

Add start/end paragraph alignment.

In the future, when the paragraph direction is not explicitly set,
start/end options will allow alignment to change line by line,
whereas left/right alignment will be constant.

Add centre/right paragraph alignment.

Internally mark hard line breaks.

Internally allow splitting texts at endpoints.

This is required in order to preserve a hard line break at the end of
the input.

Separate split list functions into a module.

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.

Remove TODO which would require an API break.

Simplify code since run direction is always known.

Add TODO for end-of-line whitespace.

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.

Fix documentation: internal references.