draft: Apply italic correction

This is a work-in-progress attempt to implement Italic correction between runs of different slant.

To merge this without regressing on accurate accounting for line widths, we need to do ltr reordering on-the-fly, so we know which runs will be visually adjacent as we add them to the line.

Behdad pointed at https://github.com/fribidi/linear-reorder for an implementation of rtl reordering that could be useful here.