Fix slowdown by moving overlays less often.

Now a new function `hl-paren-initiate-highlight` is in `post-command-hook`
instead of `hl-paren-highlight` itself.  The new function uses a timer to skip
calls to `hl-paren-highlight` in case those come faster than about one in a
quarter of a second.  That easily happens when scrolling by pressing and holding
`C-n`.

Fixes issue #8.

In contrast to the subject of the issue, I don't use an idle timer but a normal
one to ensure that the hl-paren updates are immediately visible from a user's
point of view.

highlight-parentheses.el

@@ -96,6 +96,9 @@ Color attributes might be overriden by `hl-paren-colors' and
 This is used to prevent analyzing the same context over and over.")
 (make-variable-buffer-local 'hl-paren-last-point)
 
+(defvar hl-paren-timer nil
+  "A timer initiating the movement of the `hl-paren-overlays'.")
+
 (defun hl-paren-highlight ()
   "Highlight the parentheses around point."
   (unless (= (point) hl-paren-last-point)
@@ -109,13 +112,18 @@ This is used to prevent analyzing the same context over and over.")
                         (cdr overlays))
               (move-overlay (pop overlays) pos1 (1+ pos1))
               (when (setq pos2 (scan-sexps pos1 1))
-                (move-overlay (pop overlays) (1- pos2) pos2)
-                ))
+                (move-overlay (pop overlays) (1- pos2) pos2)))
           (error nil))
         (goto-char pos))
       (dolist (ov overlays)
         (move-overlay ov 1 1)))))
 
+(defun hl-paren-initiate-highlight ()
+  "Move the `hl-paren-overlays' after a short fraction of time."
+  (when hl-paren-timer
+    (cancel-timer hl-paren-timer))
+  (setq hl-paren-timer (run-at-time 0.23 nil #'hl-paren-highlight)))
+
 ;;;###autoload
 (define-minor-mode highlight-parentheses-mode
   "Minor mode to highlight the surrounding parentheses."
@@ -123,10 +131,10 @@ This is used to prevent analyzing the same context over and over.")
   (mapc 'delete-overlay hl-paren-overlays)
   (kill-local-variable 'hl-paren-overlays)
   (kill-local-variable 'hl-paren-last-point)
-  (remove-hook 'post-command-hook 'hl-paren-highlight t)
+  (remove-hook 'post-command-hook 'hl-paren-initiate-highlight t)
   (when highlight-parentheses-mode
     (hl-paren-create-overlays)
-    (add-hook 'post-command-hook 'hl-paren-highlight nil t)))
+    (add-hook 'post-command-hook 'hl-paren-initiate-highlight nil t)))
 
 ;;;###autoload
 (define-globalized-minor-mode global-highlight-parentheses-mode