Add forceLeftAlign function for testing.

Generalise shiftFragment function.

Intended for testing paragraphSafeWidth.

Allow explicit control of box collapsing.

Allow generating an empty run.

Add extensible filter for invisible lines.

Store default level in BiDiLevels.

This will be required for handling empty text sequences.

Prepare for vertical-align in BoxOptions.

Implement calculation of safe paragraph width.

Refactor lineNumbers into a common module.

Allow paragraphRect without point 0,0.

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.

Internally support text breaks in ascending order.

Intended as a workaround to `breaksRight` not including the status of
the end-of-text break.

Separate split list functions into a module.

Fix terminology for box layout bounds.

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.