Use `org-preview-latex-image-directory' instead of hardcoding

* lisp/ox-odt.el (org-odt--translate-latex-fragments):
* lisp/ox-html.el (org-html--format-image): Use
`org-preview-latex-image-directory' instead of hardcoding the
directory name.

See <https://orgmode.org/list/87a70v9jan.fsf@ucl.ac.uk>

Reported-by: Eric S Fraga <e.fraga@ucl.ac.uk>

lisp/ox-html.el

@@ -1638,7 +1638,8 @@ a communication channel."
      (org-html--make-attribute-string
       (org-combine-plists
        (list :src source
-	     :alt (if (string-match-p "^ltxpng/" source)
+	     :alt (if (string-match-p
+		       (concat "^" org-preview-latex-image-directory) source)
 		      (org-html-encode-plain-text
 		       (org-find-text-property-in-string 'org-latex-src source))
 		    (file-name-nondirectory source)))

lisp/ox-odt.el

@@ -3730,7 +3730,8 @@ contextual information."
 		 (cache-dir (file-name-directory input-file))
 		 (cache-subdir (concat
 				(cl-case processing-type
-				  ((dvipng imagemagick) "ltxpng/")
+				  ((dvipng imagemagick)
+				   org-preview-latex-image-directory)
 				  (mathml "ltxmathml/"))
 				(file-name-sans-extension
 				 (file-name-nondirectory input-file))))