bug#26959: Feature request: bold underlines

[Drew Adams <drew.adams@oracle.com>]

> > See my reply to bug #26958.  Should we have two requests:
> > 1. Wavy (and plain) underline to follow text scaling (#26958).
> > 2. Be able to configure underline line-width (#26959).
> 
> Yes, that's a great summary.
> 
> > Is #2 enough?  If we could customize the :line-width for :underline
> > (and :overline? and :strike-through?), would that be sufficient, or
> > is there also a need for such lines to be sensitive to text scaling?
> 
> I don't think so — sensitivity to text scaling would be a useful, distinct
> feature.  Otherwise Lisp code will have to inspect font sizes and adjust
> thickness based on it, which will cause inconsistencies.  Additionally, the
> amount of space between the text and the underline needs to be adjusted when
> the font size changes — and this is currently done on GNU/Linux already.

Whatever decision is made about what the most appropriate behavior
is, should we make it optional, e.g., give users a way to _not_
scale such lines, boxes, etc.?