org.el (org-format-latex-header): Put DEFAULT-PACKAGES before PACKAGES

* lisp/org.el (org-format-latex-header): Put DEFAULT-PACKAGES before
PACKAGES, as per org-latex-packages-alist's documentation.

`org-format-latex-header' is mostly used to generate in-buffer images
from LaTeX fragments.  For LaTeX document export, the header is
generated by `org-splice-latex-header' ond `org-latex-classes' instead
and the default and documented behaviour is to insert DEFAULT-PACKAGES
before PACKAGES.

See also
https://list.orgmode.org/877d5gg5rt.fsf@localhost/T/#m2ad2f3b1509e1af72016e8e6fad3557ff3083046

lisp/org.el

@@ -3248,8 +3248,8 @@ images at the same place."
 
 (defcustom org-format-latex-header "\\documentclass{article}
 \\usepackage[usenames]{color}
-\[PACKAGES]
 \[DEFAULT-PACKAGES]
+\[PACKAGES]
 \\pagestyle{empty}             % do not remove
 % The settings below are copied from fullpage.sty
 \\setlength{\\textwidth}{\\paperwidth}