bug#51704: 29.0.50; set-fontset-font do not work well with :weight 'normal of font-spec

[Eli Zaretskii <eliz@gnu.org>]

> From: Lars Ingebrigtsen <larsi@gnus.org>
> Cc: tumashu@163.com,  51704@debbugs.gnu.org
> Date: Wed, 10 Nov 2021 00:45:16 +0100
> 
> Eli Zaretskii <eliz@gnu.org> writes:
> 
> > Can we rethink this incompatible change?  Isn't there a way of
> > achieving the same goal without breaking people's font customizations?
> > Why was there a need to make 'normal' and 'medium' be different?
> 
> Because (some) fonts define both a `normal' and `medium' weight, and
> they are different weights.  Previously Emacs couldn't use both.
> 
> The basic problem is in the font selection mechanism we choose -- if
> we're asking for `normal' (weight 80) and the font doesn't have a weight
> in the area of 78-82 (I think it was; it's been a couple of weeks since
> I looked at this), it'll just give up and say "I can't find the font"
> instead of choosing the closest weight the font does have.  Which would
> improve the situation significantly for users.

But the problem I saw was that w32font.c didn't have both medium and
normal, it had only one of them.  So I now readjusted the conversion
between Fontconfig values and MS-Windows values in a way that supports
both normal and medium.  Feng, could you please try the latest master
branch, with your original fontset customizations, and see if there is
any change (hopefully, for the better)?