Fix animated gifs (Bug#6981).

Fix animated gifs (Bug#6981).

* lisp/image-mode.el (image-toggle-display-image): Ensure that the
image spec passed to the animate timer is the same object as in
the the buffer's display property.
(image-transform-properties): Doc fix.

* lisp/image.el (image-animate-max-time): Default to nil.

* lisp/image.el (image-animate-max-time): Allow nil and t values.
Default to nil.
(create-animated-image): Doc fix.
(image-animate-start): Remove second arg; just use
image-animate-max-time.
(image-animate-timeout): Doc fix.  Args changed.