bug#67810: 29.1; fonts use synthetic bold on Linux / pgtk

[Tim Ruffing]

On Thu, 2023-12-14 at 14:55 -0800, Stefan Kangas wrote:
> 
> They should use .svg files, I think.  See the branch scratch/icons
> for
> the basic approach.

Yeah, that's a very clean approach, of course, but it's restricted to
graphical displays. And consideration went into the current design
described in 
https://www.gnu.org/software/emacs/manual/html_node/elisp/Icons.html
which says "Icons should also have a textual fallback." and which has
an example with "textual" symbols. 

@Po Lu:
Independent of icons, I still think that overstriking is a bit
unexpected. (I mean, even Eli didn't know about it.) I see that a font
regex is too much, but do you think a simple boolean option would be a
good idea? Or do you think the current behavior should simply be
documented more prominently?

Tim