Ensure LaTeX equation environments are processed by pandoc.

Works around a bug in pandoc (jgm/pandoc#1764,
present in at least up-to and including pandoc 1.18) to surround every
AMSMath latex environment in the org file with plain TeX equation block
delimiters, '$$ .. $$' in order for pandoc to properly recognise the
maths environment as a latex equation.  Also adds surrounding
line-breaks so that pandoc treats the math environment as its own
paragraph.  This avoids having text before or after the math
environment ending up on the same line as the equation.

ox-pandoc.el

@@ -211,7 +211,8 @@
  :type 'list)
 
 (org-export-define-derived-backend 'pandoc 'org
- :translate-alist '((link . org-pandoc-link)
+ :translate-alist '((latex-environment . org-pandoc-latex-environ)
+ (link . org-pandoc-link)
  (template . org-pandoc-template)
  (paragraph . org-pandoc-identity))
  ;; :export-block "PANDOC"
@@ -1247,12 +1248,54 @@ t means output to buffer."
  (concat (make-temp-name ".tmp") ".org") s)
  a s v b e (lambda (f) (org-pandoc-run-to-buffer-or-file f format s buf-or-open))))
 
-(defun org-pandoc-link (link contents info)
+(defun org-pandoc-latex-environ (blob contents _info)
+ "Transcode a latex environment for export with pandoc.
+Works around a bug in
+pandoc (https://github.com/jgm/pandoc/issues/1764, present in at
+least up-to and including pandoc 1.18) to surround an AMSMath
+latex environment in BLOB with plain TeX equation block
+delimiters, '$$ .. $$' in order for pandoc to properly recognise
+the maths environment as a latex equation. Also adds surrounding
+line-breaks so that pandoc treats the math environment as its own
+paragraph. This avoids having text before or after the math
+environment ending up on the same line as the equation.
+CONTENTS is its contents, as a string or nil. INFO is ignored."
+ (let ((raw-value (org-export-expand blob contents t)))
+ ;; If we're exporting to a TeX-based format, there's no need for
+ ;; this hack
+ (if (member org-pandoc-format '(beamer beamer-pdf latex latex-pdf))
+ raw-value
+ ;; Otherwise, add '$$' elements before and after the block to
+ ;; get pandoc to process it.
+ (let* ((post-blank (org-element-property :post-blank blob))
+ (case-fold-search t)
+ (preface
+ (replace-regexp-in-string
+ (rx (group-n 1 "\\begin{"
+ (or "align" "alignat" "eqnarray" "equation" "flalign" "gather" "multline")
+ (zero-or-one "*" ) "}"))
+ "\n$$\\1"
+ raw-value))
+ (output
+ (replace-regexp-in-string
+ (rx (group-n 1 "\\end{"
+ (or "align" "alignat" "eqnarray" "equation" "flalign" "gather" "multline")
+ (zero-or-one "*" ) "}"))
+ "\\1$$"
+ preface)))
+ ;; If we've added the '$$' delimiters, then also set the
+ ;; :post-blank property to add a blank line after this current
+ ;; latex equation environment
+ (unless (or (>= post-blank 1)
+ (string-equal raw-value output))
+ (org-element-put-property blob :post-blank 1))
+ output))))
+
+(defun org-pandoc-link (link contents _info)
  "Transcode LINK object using the registered formatter for the
-'pandoc backend. If none exists, transcode using the registered
-formatter for the 'org export backend. CONTENTS is the
-description of the link, as a string, or nil. INFO is a plist
-containing current export state."
+'pandoc backend. If none exists, transcode using the registered
+formatter for the 'org export backend. CONTENTS is the
+description of the link, as a string, or nil. INFO is ignored."
  (or (org-export-custom-protocol-maybe link contents 'pandoc)
  (org-export-custom-protocol-maybe link contents 'org)
  (org-element-link-interpreter link contents)))
@@ -1308,12 +1351,11 @@ Option table is created in this stage."
  (funcall org-template contents info)
  contents)))
 
-(defun org-pandoc-identity (blob contents info)
+(defun org-pandoc-identity (blob contents _info)
  "Transcode BLOB element or object back into Org syntax.
-CONTENTS is its contents, as a string or nil. INFO is ignored.
+CONTENTS is its contents, as a string or nil.  INFO is ignored.
 Like `org-org-identity', but also preserves #+ATTR_* tags in the
 output."
- (ignore info)
  (org-export-expand blob contents t))
 
 (defun org-pandoc-put-options (options)