Make `n' in image-mode work more reliably with external formats

* lisp/image-mode.el (image-mode): Init the external machinery so
that commands like `n' work for those files.
* lisp/image/image-converter.el (image-converter-initialize):
Factored out into own function.
(image-convert-p): Use it.
(image-convert): Ditto.

lisp/image-mode.el

@@ -625,6 +625,8 @@ image as text, when opening such images in `image-mode'."
 
 (put 'image-mode 'mode-class 'special)
 
+(declare-function image-converter-initialize "image-converter.el")
+
 ;;;###autoload
 (defun image-mode ()
   "Major mode for image files.
@@ -650,7 +652,12 @@ Key bindings:
                        "Empty file"
                      "(New file)")
                  "Empty buffer"))
-    (image-mode--display)))
+    (image-mode--display)
+    ;; Ensure that we recognize externally parsed image formats in
+    ;; commands like `n'.
+    (when image-use-external-converter
+      (require 'image-converter)
+      (image-converter-initialize))))
 
 (defun image-mode--display ()
   (if (not (image-get-display-property))

lisp/image/image-converter.el

@@ -68,15 +68,19 @@ not, conversion will fail."
     (imagemagick :command "convert" :probe ("-list" "format")))
   "List of supported image converters to try.")
 
+(defun image-converter-initialize ()
+  "Determine the external image converter to be used.
+This also determines which external formats we can parse."
+  (unless image-converter
+    (image-converter--find-converter)))
+
 (defun image-convert-p (source &optional data-p)
   "Return `image-convert' if SOURCE is an image that can be converted.
 SOURCE can either be a file name or a string containing image
 data.  In the latter case, DATA-P should be non-nil.  If DATA-P
 is a string, it should be a MIME format string like
 \"image/gif\"."
-  ;; Find an installed image converter.
-  (unless image-converter
-    (image-converter--find-converter))
+  (image-converter-initialize)
   ;; When image-converter was customized
   (when (and image-converter (not image-converter-regexp))
     (when-let ((formats (image-converter--probe image-converter)))
@@ -111,9 +115,7 @@ IMAGE can also be an image object as returned by `create-image'.
 
 This function converts the image the preferred format, and the
 converted image data is returned as a string."
-  ;; Find an installed image converter.
-  (unless image-converter
-    (image-converter--find-converter))
+  (image-converter-initialize)
   (unless image-converter
     (error "No external image converters available"))
   (when (and image-format