org-latex-preview: Split org-latex-preview--create

* lisp/org-latex-preview.el (org-latex-preview--create): This function
does many tasks, including computing face colors and hashes for
fragments.  Split this function into three to reuse the parts for other
planned purposes, such as clearing cached images.
(org-latex-preview--colors-at): Finds appropriate foreground and
background colors at a position for the preview image of a LaTeX
fragment.  Also improve the code to handle multiple faces.
(org-latex-preview--hash): Computes the hash key for a LaTeX fragment
combined with relevant processing metadata.  This key is used to
identify the fragment when caching and placing images.

lisp/org-latex-preview.el

@@ -814,40 +814,23 @@ Some of the options can be changed using the variable
          fragment-info prev-fg prev-bg)
     (save-excursion
       (dolist (element elements)
-        (let* ((beg (org-element-property :begin element))
-               (end (save-excursion
-                      (goto-char (org-element-property :end element))
-                      (skip-chars-backward " \r\t\n")
-                      (point)))
-               (value (org-element-property :value element))
-               (face (or (and (> beg 1)
-                              (get-text-property (1- beg) 'face))
-                         'default))
-               (fg (pcase (plist-get org-latex-preview-options :foreground)
-                     ('auto (face-attribute face :foreground nil 'default))
-                     ('default (face-attribute 'default :foreground nil))
-                     (color color)))
-               (bg (pcase (plist-get org-latex-preview-options :background)
-                     ('auto (face-attribute face :background nil 'default))
-                     ('default (face-attribute 'default :background nil))
-                     (color color)))
-               (hash (sha1 (prin1-to-string
-                            (list processing-type
-                                  org-latex-preview-preamble
-                                  org-latex-default-packages-alist
-                                  org-latex-packages-alist
-                                  org-latex-preview-options
-                                  value
-                                  (if (equal imagetype "svg")
-                                      'svg fg)
-                                  bg))))
-               (options (org-combine-plists
-                         org-latex-preview-options
-                         (list :foreground fg
-                               :background bg
-                               :continue-color
-                               (and (equal prev-bg bg)
-                                    (equal prev-fg fg))))))
+        (pcase-let* ((beg (org-element-property :begin element))
+                     (end (- (org-element-property :end element)
+                             (or (org-element-property :post-blank element) 0)
+                             (if (eq (char-before (org-element-property :end element))
+                                     ?\n)
+                                 1 0)))
+                     (value (org-element-property :value element))
+                     (`(,fg ,bg) (org-latex-preview--colors-at beg))
+                     (hash (org-latex-preview--hash
+                            processing-type value imagetype fg bg))
+                     (options (org-combine-plists
+                               org-latex-preview-options
+                               (list :foreground fg
+                                     :background bg
+                                     :continue-color
+                                     (and (equal prev-bg bg)
+                                          (equal prev-fg fg))))))
           (if-let ((path-info (org-latex-preview--get-cached hash)))
               (org-latex-preview--update-overlay
                (org-latex-preview--make-overlay beg end)
@@ -863,6 +846,51 @@ Some of the options can be changed using the variable
        processing-type
        (nreverse fragment-info)))))
 
+(defun org-latex-preview--colors-at (pos)
+  "Find colors for LaTeX previews to be inserted at POS."
+  (let* ((face (or (and (> pos 1)
+                        (get-text-property (1- pos) 'face))
+                   'default))
+         (faces (if (consp face)
+                    (append face '(default))
+                  (list face 'default)))
+         (fg (pcase (plist-get org-latex-preview-options :foreground)
+               ('auto (face-attribute (car faces) :foreground nil (cdr faces)))
+               ('default (face-attribute 'default :foreground nil))
+               (color color)))
+         (bg (pcase (plist-get org-latex-preview-options :background)
+               ('auto (face-attribute (car faces) :background nil (cdr faces)))
+               ('default (face-attribute 'default :background nil))
+               (color color))))
+    (list fg bg)))
+
+(defun org-latex-preview--hash (processing-type string imagetype fg bg)
+  "Return a SHA1 hash for referencing LaTeX fragments when previewing them.
+
+PROCESSING-TYPE is the type of process used to create the
+preview, see `org-latex-preview-default-process'.
+
+PREAMBLE is the LaTeX preamble used in the generated LaTeX document.
+
+STRING is the string to be hashed, typically the contents of a
+LaTeX fragment.
+
+IMAGETYPE is the type of image to be created, see
+`org-latex-preview-process-alist'.
+
+FG and BG are the foreground and background colors for the
+image."
+  (sha1 (prin1-to-string
+         (list processing-type
+               org-latex-preview-preamble
+               org-latex-default-packages-alist
+               org-latex-packages-alist
+               org-latex-preview-options
+               string
+               (if (equal imagetype "svg")
+                   'svg fg)
+               bg))))
+
 (defun org-latex-preview-create-image-async (processing-type fragments-info)
   "Preview PREVIEW-STRINGS asynchronously with method PROCESSING-TYPE.