I find a new tool: dvisvgm (
https://github.com/mgieseki/dvisvgm) ,which we can use it to preview latex fregments, it can deal with
xetex's xdv file as well (it will available in texlive2016).
The questions are:
1. which path we should use: "dvi/xdv -> svg" or "dvi/xdv -> svg -> png" ?
2. if we use the first path, how to deal with org-latex-preview-ltxpng-directory? should we rename it to org-latex-preview-ltximg-directory or
org-latex-preview-ltxpic-directory ?