Set org-latex-preview-ltxpng-directory

Set org-latex-preview-ltxpng-directory to "~/tmp/ltxpng/".

The default value is "ltxpng/". Because this is a relative path org will
create a "ltxpng" folder alongside each org-mode file when generating
the previews. By changing to "~/tmp/ltxpng/" it will always create the
previews in the same folder.