org.el (org-create-formula-image): Fix ignored background color

* lisp/org.el (org-preview-latex-process-alist): add a
`:transparent-image-converter' property for `dvipng'.
(org-create-formula-image): If available, use
`:transparent-image-converter' when no background color is set.

lisp/org.el

@@ -3324,7 +3324,9 @@ All available processes and theirs documents can be found in
      :image-output-type "png"
      :image-size-adjust (1.0 . 1.0)
      :latex-compiler ("latex -interaction nonstopmode -output-directory %o %f")
-     :image-converter ("dvipng -D %D -T tight -bg Transparent -o %O %f"))
+     :image-converter ("dvipng -D %D -T tight -o %O %f")
+     :transparent-image-converter
+     ("dvipng -D %D -T tight -bg Transparent -o %O %f"))
     (dvisvgm
      :programs ("latex" "dvisvgm")
      :description "dvi > svg"
@@ -3379,6 +3381,9 @@ PROPERTIES accepts the following attributes:
                       given to the shell and supports any of the following
                       place-holders defined below.
 
+If set, :transparent-image-converter is used instead of :image-converter to
+convert an image when the background color is nil or \"Transparent\".
+
 Place-holders used by `:image-converter' and `:latex-compiler':
 
   %f    input file name
@@ -16303,7 +16308,6 @@ a HTML file."
 	       org-format-latex-header
 	       'snippet)))
 	 (latex-compiler (plist-get processing-info :latex-compiler))
-	 (image-converter (plist-get processing-info :image-converter))
 	 (tmpdir temporary-file-directory)
 	 (texfilebase (make-temp-name
 		       (expand-file-name "orgtex" tmpdir)))
@@ -16317,7 +16321,11 @@ a HTML file."
 		 "Black"))
 	 (bg (or (plist-get options (if buffer :background :html-background))
 		 "Transparent"))
-	 (log-buf (get-buffer-create "*Org Preview LaTeX Output*"))
+	 (image-converter
+          (or (and (string= bg "Transparent")
+                   (plist-get processing-info :transparent-image-converter))
+              (plist-get processing-info :image-converter)))
+         (log-buf (get-buffer-create "*Org Preview LaTeX Output*"))
 	 (resize-mini-windows nil)) ;Fix Emacs flicker when creating image.
     (dolist (program programs)
       (org-check-external-command program error-message))