bug#66941: 29.1; Emacs on Windows can't display CJK characters correctly in svg images

[Eli Zaretskii <eliz@gnu.org>]

> Date: Sun, 5 Nov 2023 23:27:23 +0000
> From: Alan Third <alan@idiocy.org>
> Cc: ichernyshovvv@gmail.com, 66941@debbugs.gnu.org
> 
> On Sun, Nov 05, 2023 at 08:54:27AM +0200, Eli Zaretskii wrote:
> > > Cc: 66941@debbugs.gnu.org
> > > Date: Sun, 05 Nov 2023 08:47:08 +0200
> > > From: Eli Zaretskii <eliz@gnu.org>
> > > 
> > > > (progn
> > > >   (require 'svg)
> > > >   (switch-to-buffer "*svg-test*")
> > > >   (let ((svg-image (svg-create 200 200))
> > > > 	(inhibit-read-only t))
> > > >     (erase-buffer)
> > > >     (svg-text svg-image "我喜欢自由软件" :y 100 :x 50)
> > > >     (svg-print svg-image)
> > > >     (image-mode)))
> > > 
> > > I cannot get Emacs on Windows to display _any_ non-ASCII text with the
> > > above recipe, not even if I use Latin-1 characters or characters from
> > > the current system codepage.  Emacs encodes non-ASCII characters as
> > > &#nnnn numbers (you can see that if you type "C-c C-c" in the
> > > *svg-test* buffer), which is probably correct, but the SVG image is
> > > not displayed for some reason.
> > > 
> > > Alan, any ideas or suggestions?
> > 
> > Could be a problem with the font used by librsvg to show text?  Maybe
> > the font doesn't have any non-ASCII characters.
> 
> This seems plausible. It should be possible to check by over-riding
> the CSS with a font you know should work:
> 
> (progn
>   (require 'svg)
>   (switch-to-buffer "*svg-test*")
>   (let ((svg-image (svg-create 200 200))
>         (inhibit-read-only t))
>     (erase-buffer)
>     (svg-text svg-image "我喜欢自由软件" :y 100 :x 50)
>     (insert-image (svg-image svg-image :css "svg{font-family:\"Noto Sans Mono CJK HK\";}"))))
> 
> Choose a font you know can show the text you're testing. I've a vague
> memory that the Noto CJK fonts behave slightly oddly on Windows, so
> maybe something else is a better choice.
> 
> I think setting the :css attribute to an empty string should reset
> librsvg so it will use its default font settings and that would let
> us see if it works right without our customisation.

Thanks.  I tried the above, and it didn't work, at least with (old)
librsvg 2.40.1 I have here.  Moreover, if I use a plain ASCII string,
which does display, the font used for display is identical whether or
not I specify font-family in the :css attribute, so I think somehow
this attribute doesn't have any effect.

Is it possible to specify a font rather than font-family?  Font
families are treated poorly on MS-Windows, so I'd prefer to specify a
particular font, to be sure.