Caret x positions for this line: positions[col] for
col in 0..text.length (so it always has text.length + 1 entries).
Index into the source text of this line's first character.
The line's visible text (never contains the newline).
Rendered width — positions[text.length].
Top of the line box.
True when the line ends at a
\nor the end of the text (not a soft wrap).