bug#47074: Replace XPM icons with Unicode codepoints in customize/widgets

[Eli Zaretskii]

> From: Stefan Kangas <stefan@marxist.se>
> Date: Fri, 12 Mar 2021 20:44:55 -0600
> Cc: 47074@debbugs.gnu.org
> 
> @@ -2385,9 +2385,9 @@ 'checkbox
>    ;; We could probably do the same job as the images using single
>    ;; space characters in a boxed face with a stretch specification to
>    ;; make them square.
> -  :on-glyph "checked"
> +  :on-glyph "ui/checkbox-checked"
>    :off "[ ]"
> -  :off-glyph "unchecked"
> +  :off-glyph "ui/checkbox"
>    :help-echo "Toggle this item."
>    :action 'widget-checkbox-action)

How will this work in an Emacs session that doesn't support SVG?  The
images/ui/ directory has only SVG images.  Am I missing something?