babel latex headers and image generation commands

[Matt Huszagh <huszaghmatt@gmail.com>]

I spent some time today trying to get latex babel source blocks to work
for me and discovered that calling `org-babel-execute:latex` ignores the
:headers header if the output file is a png without setting imagemagick
to t. It's easy to see this in the source code: the conditions mentioned
above leads to calling org-create-formula-image without passing in the
headers. I think this is a bug, although maybe I missed it somewhere in
the documentation? Here's a MWE if you want it

#+header: :file "test.png"
#+header: :headers '("\\def\\hello{hello}")
#+begin_src latex :results output file link
\hello
#+end_src

doesn't work, but

#+header: :file "test.png"
#+header: :imagemagick t
#+header: :headers '("\\def\\hello{hello}")
#+begin_src latex :results output file link
\hello
#+end_src

does.

However, this got me thinking that I wish executing latex blocks behaved
a bit more like latex fragment previews. Particularly, the ability to
customize `image-converter`. So, I'm thinking about adding a
customization option that allows a user to use the image-converter
portion of an existing org-preview-latex-process-alist entry (I guess
the most obvious choice would be
org-preview-latex-default-process). Although I guess I could just add a
new variable like what we have with org-latex-pdf-process. Maybe that's
better since we'd only be using one part of the
latex-process-alist. What are people's thoughts on this?