Allow stopping animations automatically when the image disappears

Allow stopping animations automatically when the image disappears

* lisp/image.el (image-animate): Allow the animation to be stopped
automatically when the image is removed from the buffer.
(image-animate-timeout): Stop the animation if the image is
removed (and that has been requested).

* lisp/net/shr.el (shr-put-image): Stop animations if the image is
removed.