[O] org-create-formula-image-with-imagemagick conflicts with org-latex-pdf-process

[Florian Beck]

`org-create-formula-image-with-imagemagick' doesn't handle the case that
`org-latex-pdf-process' is set to a function.

To reproduce:

 - set `org-latex-pdf-process' to a function
 - (setq org-latex-create-formula-image-program 'imagemagick)
   this makes preview fail
 - set #+OPTIONS: latex:imagemagick => html export broken

However, just calling this function from
`org-create-formula-image-with-imagemagick' would not be a good idea.
How about adding a new variable `org-latex-formula-pdf-process'?

-- 
Florian Beck