bug#57813: Icon images are non-functional

[Juri Linkov]

> (info "(elisp) Icons") suggests to use:
>
>      (define-icon outline-open button
>        '((image "right.svg" "open.xpm" "open.pbm" :height line)
>          (emoji "▶️")
>          (symbol "▶" "➤")
>          (text "open" :face icon-button))
>        "Icon used for buttons for opening a section in outline buffers."
>        :version "29.1"
>        :help-echo "Open this section")
>
> So I tried this with 'C-h b', but images are displayed as black boxes.
>
> PS: "right.svg" exists in etc/images/right.svg.

Fixed now in 471414fe6b.