Bug: Can't set background color of latex fragment

[Sébastien Miquel]

To reproduce with `emacs -Q` :
 - Open an org buffer
 - Call ~(setq org-format-latex-options '(:foreground default
   :background "Black" :matchers ("$")))~
 - Call =C-c C-x C-l= (org-latex-preview) on a latex fragment such as
   $abc$

This bug was introduced by the commit 2f9e1569f which adds the option
`-bg Transparent` to the arguments of `dvipng`. According to its
manual, this option should be ignored if a background is already set,
but it doesn't seem to be. Perhaps org should set it differently.

--
Sébastien Miquel