bug#30684: Add image-animate-image

[積丹尼 Dan Jacobson]

(info "(emacs) File Conveniences") says
       If the image can be animated, the command <RET>
    (‘image-toggle-animation’) starts or stops the animation.

OK but many people would like their animated images animated by default
so they don't have to press RET.

So
(setq
  image-animate-image t ; New variable. Please add.
  image-animate-loop t)
would emulate browsers. And RET would now stop and start, instead of
start and stop.