Option to chose directory for latex preview images

* org.el (org-latex-preview-ltxpng-directory): New option.
  (org-preview-latex-fragment): Store LaTeX preview images in
  `org-latex-preview-ltxpng-directory'.

Added an option `org-latex-preview-ltxpng-directory' with a path
where the preview images will be stored.  The default does not
change (small little ltxpng scattered all over the place) but now
the user can chose an absolute path and put all the images on a
single place.

TINYCHANGE

lisp/org.el

@@ -3419,6 +3419,14 @@ imagemagick     Convert the LaTeX fragments to pdf files and use imagemagick
 	  (const :tag "dvipng" dvipng)
 	  (const :tag "imagemagick" imagemagick)))
 
+(defcustom org-latex-preview-ltxpng-directory "ltxpng/"
+  "Path to store latex preview images. A relative path here creates many
+   directories relative to the processed org files paths. An absolute path
+   puts all preview images at the same place."
+  :group 'org-latex
+  :version "24.2"
+  :type 'string)
+
 (defun org-format-latex-mathml-available-p ()
   "Return t if `org-latex-to-mathml-convert-command' is usable."
   (save-match-data
@@ -17163,8 +17171,8 @@ The images can be removed again with \\[org-ctrl-c-ctrl-c]."
 	(narrow-to-region beg end)
 	(goto-char beg)
 	(org-format-latex
-	 (concat "ltxpng/" (file-name-sans-extension
-			    (file-name-nondirectory
+	 (concat org-latex-preview-ltxpng-directory (file-name-sans-extension
+			     (file-name-nondirectory
 			     buffer-file-name)))
 	 default-directory 'overlays msg at 'forbuffer
 	 org-latex-create-formula-image-program)