Reduce use of double variables in Row

All the code that is run before row metrics have been computed should use int arithmetic. After metrics have been computed, we still need doubles because fully justified rows use double for Row::Element::extra.

Rename Row::x to Row::left_margin and change its type to int.

Rename Row::Element::width() to full_width(). In some places of the code, use dim.wid (the int version without the extra separator) because metrics have not been computed.

Let Row::Element::x2pos take a int& argument instead of double&

Let Row::Element::breakAt take a int argument instead of double