bug#65191: 29.1; -ms and -cr CL options don't work

[Po Lu via "Bug reports for GNU Emacs, the Swiss army knife of text editors" <bug-gnu-emacs@gnu.org>]

Eli Zaretskii <eliz@gnu.org> writes:

> I don't think I understand this well enough to discuss this
> intelligently, so please elaborate on what happens, and include
> pointers to more of the code so I and others could complement the
> descriptions with what's actually in the code, and maybe suggest
> alternative ways of dealing with this.
>
> Not responding to Cairo setting changes sounds like a loss: what
> features are currently based on that, and would be lost if we revert
> this support?  Also, can't you think about an alternative way of
> handling dynamic settings that will let us eat the cake, but also have
> it?

The problem boils down to apply_xft_settings, within xsettings.c.  Under
Cairo, that function calls:

  FcConfigSubstitute (NULL, pat, FcMatchPattern);
  options = cairo_font_options_create ();
  ftcrfont_get_default_font_options (dpyinfo, options); <------
  cairo_ft_font_options_substitute (options, pat);
  cairo_font_options_destroy (options);
  FcDefaultSubstitute (pat);

to retrieve the settings currently in effect, and compare them against
the updated settings within the XSettings event.  If a divergence is
located, a config-changed event is sent.  Under Xft, the following code
is ultimately run:

      XftDefaultSet (dpyinfo->display, pat);

but Cairo provides no analog, so these settings are never saved back to
Cairo (nor can I find any means by which to do so.)