resurrect org-create-formula-image

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.