Add functions to change the speed of animated images

* lisp/image-mode.el (image-mode-map): Add menu items to reverse,
increase, decrease, reset animation speed.
(image--set-speed, image-increase-speed, image-decrease-speed)
(image-reverse-speed, image-reset-speed): New functions.
(image-mode-map): Add bindings for speed commands.

* lisp/image.el (image-animate-get-speed, image-animate-set-speed):
New functions.
(image-animate-timeout): Respect image :speed property.

* etc/NEWS: Mention this.

1 files changed, 4 insertions, 0 deletions

diff --git a/etc/NEWS b/etc/NEWS
index ef76f308303..8bf72ddfedd 100644
--- a/etc/NEWS
+++ b/etc/NEWS
@@ -347,6 +347,8 @@ directory, respectively.
 `f' (`image-next-frame') and `b' (`image-previous-frame') visit the
 next or previous frame.  `F' (`image-goto-frame') shows a specific frame.
 
+*** New commands to speed up, slow down, or reverse animation.
+
 ---
 *** The command `image-mode-fit-frame' deletes other windows.
 When toggling, it restores the frame's previous window configuration.
@@ -683,6 +685,8 @@ Emacs uses `image-default-frame-delay'.
 *** New functions `image-current-frame' and `image-show-frame' for getting
 and setting the current frame of a multi-frame image.
 
+*** You can change the speed of animated images.
+
 ** Changes in encoding and decoding of text
 
 ---