If gnus-visual is nil, don't fontify patches and the like

* doc/misc/emacs-mime.texi (Display Customization): Document it.

* lisp/gnus/gnus-art.el (gnus-mime-display-single): Bind it.

* lisp/gnus/mm-view.el (mm-inline-font-lock): New variable (bug#38421).
(mm-display-inline-fontify): Use it.

doc/misc/emacs-mime.texi

@@ -472,6 +472,13 @@ the case if you save it to disk and launch it in a different way
 to launch any external programs, set this variable to @code{nil} or
 @code{ask}.
 
+@item mm-inline-font-lock
+@vindex mm-inline-font-lock
+If non-@code{nil}, inlined parts that support font locking (for
+instance, patches or code snippets) will be font-locked.  This may be
+overriden by callers that have their own ways of enabling/inhibiting
+font locking.
+
 @end table
 
 @node Files and Directories

etc/NEWS

@@ -231,6 +231,12 @@ was sent.  To restore the original behavior of dating a message
 from when it is first saved or delayed, add the symbol 'Date' back to
 this user option.
 
++++
+*** New variable 'mm-inline-font-lock'.
+This variable is supposed to be bound by callers to determine whether
+inline MIME parts (that support it) are supposed to be font-locked or
+not.
+
 ** Help
 
 +++

lisp/gnus/gnus-art.el

@@ -6018,6 +6018,7 @@ If nil, don't show those extra buttons."
 (defun gnus-mime-display-single (handle)
   (let ((type (mm-handle-media-type handle))
 	(ignored gnus-ignored-mime-types)
+	(mm-inline-font-lock (gnus-visual-p 'article-highlight 'highlight))
 	(not-attachment t)
 	display text)
     (catch 'ignored

lisp/gnus/mm-view.el

@@ -59,11 +59,16 @@
   "The attributes of renderer types for text/html.")
 
 (defcustom mm-fill-flowed t
-  "If non-nil a format=flowed article will be displayed flowed."
+  "If non-nil, format=flowed articles will be displayed flowed."
   :type 'boolean
   :version "22.1"
   :group 'mime-display)
 
+;; Not a defcustom, since it's usually overridden by the callers of
+;; the mm functions.
+(defvar mm-inline-font-lock t
+  "If non-nil, do font locking of inline media types that support it.")
+
 (defcustom mm-inline-large-images-proportion 0.9
   "Maximum proportion large images can occupy in the buffer.
 This is only used if `mm-inline-large-images' is set to
@@ -502,7 +507,8 @@ If MODE is not set, try to find mode automatically."
 	      (delay-mode-hooks (set-auto-mode))
 	      (setq mode major-mode)))
 	  ;; Do not fontify if the guess mode is fundamental.
-	  (unless (eq major-mode 'fundamental-mode)
+	  (when (and (not (eq major-mode 'fundamental-mode))
+		     mm-inline-font-lock)
 	    (font-lock-ensure))))
       (setq text (buffer-string))
       (when (eq mode 'diff-mode)