Support PNG and other format callouts

[Nik Clayton <nik@nothing-going-on.demon.co.uk>]

The current callout code assumes that the images for callout bugs are
GIF format.  Here's a quick fix (assuming you want to use PNG).

  (define %callout-graphics-path%
      ;; Path to callout graphics
      "./imagelib/callouts/")

  ;; Redefine $callout-bug$ to support the %callout-graphic-ext%
  ;; variable.
  (define ($callout-bug$ conumber)
    (let ((number (if conumber (format-number conumber "1") "0")))
      (if conumber
          (if %callout-graphics%
              (if (<= conumber %callout-graphics-number-limit%)
                  (make empty-element gi: "IMG"
                        attributes: (list (list "SRC"
                                                (root-rel-path
                                                 (string-append
                                                  %callout-graphics-path%
                                                  number
                                                  %callout-graphics-ext%)))
                                          (list "HSPACE" "0")
                                          (list "VSPACE" "0")
                                          (list "BORDER" "0")
                                          (list "ALT"
                                                (string-append
                                                 "(" number ")"))))
                  (make element gi: "B"
                        (literal "(" (format-number conumber "1") ")")))
              (make element gi: "B"
                    (literal "(" (format-number conumber "1") ")")))
          (make element gi: "B"
         (literal "(??)")))))

N
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