org-mime: Fix LaTeX image bug in org-mime-org-buffer-htmlize

* contrib/lisp/org-mime.el (org-mime-replace-images): Change signature.
Always use temporary directory to create LaTeX images.
(org-mime-htmlize):
(org-mime-compose): Apply signature change above.

When exporting an org buffer with a LaTeX snippet including the Org
option "tex: dvipng" the images were always created in "/tmp/ltxpng/",
but the reported link in the htmlized buffer was the local "./ltxpng"
folder; so, unless the org file to htmlize was actually in "/tmp", the
user got an error saying that the image could not be found.

This was caused by the two different ways in which the function
`org-mime-replace-images' is used inside of
`org-mime-org-buffer-htmlize' and `org-mime-htmlize.'

To correct the problem, the function org-mime-replace-images doesn't
take anymore the current-file parameter (that was only used to get its
directory) and instead always creates a temporary directory using
temporary-file-directory.

contrib/lisp/org-mime.el

@@ -173,7 +173,7 @@ and images in a multipart/related part."
             "--" "}-<<alternative>>\n"))
     ('vm "?")))
 
-(defun org-mime-replace-images (str current-file)
+(defun org-mime-replace-images (str)
   "Replace images in html files with cid links."
   (let (html-images)
     (cons
@@ -185,7 +185,7 @@ and images in a multipart/related part."
          (let* ((url (and (string-match "src=\"\\([^\"]+\\)\"" text)
                           (match-string 1 text)))
                 (path (expand-file-name
-                       url (file-name-directory current-file)))
+                       url temporary-file-directory))
                 (ext (file-name-extension path))
                 (id (replace-regexp-in-string "[\/\\\\]" "_" path)))
            (add-to-list 'html-images
@@ -212,8 +212,6 @@ otherwise export the entire body."
                        (point-max)))
          (raw-body (concat org-mime-default-header
 			   (buffer-substring html-start html-end)))
-         (tmp-file (make-temp-name (expand-file-name
-				    "mail" temporary-file-directory)))
          (body (org-export-string-as raw-body 'org t))
          ;; because we probably don't want to export a huge style file
          (org-export-htmlize-output-type 'inline-css)
@@ -224,7 +222,7 @@ otherwise export the entire body."
          ;; to hold attachments for inline html images
          (html-and-images
           (org-mime-replace-images
-	   (org-export-string-as raw-body 'html t) tmp-file))
+	   (org-export-string-as raw-body 'html t)))
          (html-images (unless arg (cdr html-and-images)))
          (html (org-mime-apply-html-hook
                 (if arg
@@ -317,7 +315,7 @@ otherwise export the entire body."
 	     (org-export-htmlize-output-type 'inline-css)
 	     (html-and-images
 	      (org-mime-replace-images
-	       (org-export-string-as (funcall bhook body 'html) 'html t) file))
+	       (org-export-string-as (funcall bhook body 'html) 'html t)))
 	     (images (cdr html-and-images))
 	     (html (org-mime-apply-html-hook (car html-and-images))))
 	(insert (org-mime-multipart