Which cells of a terminal row changed since it was last written, as runs to rewrite instead of the whole row.

Drafter.Render.RowDamage.runs(previous_strip, strip)
#=> [{4, 3}, {40, 12}]

A run is {first_column, width}. Runs closer than :gap columns apart (default 4) are joined, since rewriting a few unchanged cells costs less than moving the cursor again. Where the runs would cover more than :whole of the row (default 0.6), or the two rows differ in width, the answer is :whole and the row is written as it is. A wide character is never split: a run that would start or end inside one takes all of it.