Divide the do_paint() loop into several subloops. The activity of
going through the line and working out which bits need to be redrawn
is now in a separate loop from the subsequent activity of actually
going through and doing the redraws. This _should_ enable me to
tinker with the which-bits-to-redraw data in between the two, thus
fixing `font-overflow'. However, I thought it would be sensible to
break the work up into two commits so we can track bugs in the
restructuring separately from bugs introduced by the new feature.