resurrect org-create-formula-image
This commit resurrects the `org-create-formula-image' function which was
removed in commit a9d3ce. This function is still called elsewhere, and
provides a simpler interface to the two backend-specific image creation
functions.
This also simplifies `org-format-latex', which still has some serious
problems such as optional arguments such as PROCESSING-TYPE which are
never assigned a default value, and extraneous variables.
At some point the `org-create-formula-image-with-imagemagick' and
`org-create-formula-image-with-dvipng' functions should be combined as a
great deal of code and logic is duplicated between the two functions.
* lisp/org.el (org-format-latex): Simplified and now makes use of the
new `org-create-formula-image' function.
(org-create-formula-image): Provides a simpler interface to the two
backend-specific functions.