Fix setting of row pos/endpos (overlapping rows)

Fix setting of row pos/endpos (overlapping rows)

In TextMetrics::breakParagraph, get rid of the fragile `pos' local
variable, which was not correctly updated. Rely on the endpos of the
last element in row instead.

Rewrite cleanupRow to rely on the endpos of last the row element to
set row endpos, instead of a `pos' parameter.