org-latex-preview: Fix bug clearing precompile cache

* lisp/org-latex-preview.el (org-latex-preview-clear-cache,
org-latex-preview--create-tex-file,
org-latex-preview--include-preview-string): LaTeX headers that are
precompiled include additional strings that set the
textwidth (according to `org-latex-preview-width') and include
preview.sty.  These strings are not appended to the header when
trying to clear the precompile cache for the present buffer.  Fix
by (i) removing the textwidth declaration from the precompiled
text, and (ii) including the preview.sty inclusion string with the
header we look for when trying to remove the corresponding fmt
file in the org-persist cache.

org-latex-preview--include-preview-string: Introduce string that
holds the text to include preview.sty in the document header.

lisp/org-latex-preview.el

@@ -1474,6 +1474,10 @@ previews."
           (and (not org-latex-preview-numbered)
                org-latex-preview--single-eqn-format)))))))
 
+(defconst org-latex-preview--include-preview-string
+  "\n\\usepackage[active,tightpage,auctex]{preview}\n"
+  "A LaTeX preamble snippet that includes preview.sty for previews.")
+
 (defun org-latex-preview--create-tex-file (processing-info fragments)
   "Create a LaTeX file based on PROCESSING-INFO and FRAGMENTS.
 
@@ -1490,13 +1494,14 @@ The path of the created LaTeX file is returned."
   (let* ((header
           (concat
            (plist-get processing-info :latex-header)
-           (let ((w org-latex-preview-width))
-             (cond
-              ((stringp w)
-               (format "\n\\setlength{\\textwidth}{%s}\n" w))
-              ((and (floatp w) (<= 0.0 w 1.0))
-               (format "\n\\setlength{\\textwidth}{%s\\paperwidth}\n" w))))
-           "\n\\usepackage[active,tightpage,auctex]{preview}\n"))
+           org-latex-preview--include-preview-string))
+         (textwidth
+          (let ((w org-latex-preview-width))
+            (cond
+             ((stringp w)
+              (format "\n\\setlength{\\textwidth}{%s}\n" w))
+             ((and (floatp w) (<= 0.0 w 1.0))
+              (format "\n\\setlength{\\textwidth}{%s\\paperwidth}\n" w)))))
          (relative-file-p
           (string-match-p "\\(?:\\\\input{\\|\\\\include{\\)[^/]" header))
          (remote-file-p (file-remote-p default-directory))
@@ -1530,7 +1535,8 @@ The path of the created LaTeX file is returned."
       ;; it is usually set during the font size intialisation that occurs at
       ;; \begin{document}.  We can either modify the \normalsize command to set
       ;; the \abovedisplayskip length, or just set it after \begin{document}.
-      (insert "\n\\begin{document}\n\n"
+      (insert textwidth
+              "\n\\begin{document}\n\n"
               "\\setlength\\abovedisplayskip{0pt}"
               " % Remove padding before equation environments.\n\n")
       (dolist (fragment-info fragments)
@@ -2124,11 +2130,12 @@ the *entire* preview cache will be cleared, and `org-persist-gc' run."
     (or org-latex-preview--preamble-content
         (setq org-latex-preview--preamble-content
               (org-latex-preview--get-preamble)))
-    (dolist (compiler org-latex-compilers)
-      (org-latex--remove-cached-preamble
-       compiler org-latex-preview--preamble-content nil)
-      (org-latex--remove-cached-preamble
-       compiler org-latex-preview--preamble-content t)))
+    (let ((full-preamble
+           (concat org-latex-preview--preamble-content
+                   org-latex-preview--include-preview-string)))
+      (dolist (compiler org-latex-compilers)
+        (org-latex--remove-cached-preamble compiler full-preamble nil)
+        (org-latex--remove-cached-preamble compiler full-preamble t))))
   (org-latex-preview-clear-overlays beg end)
   (if clear-entire-cache
       (let ((n 0))