Handle gif subimage animation delay correctly.

Handle gif subimage animation delay correctly.

* lisp/image.el (image-animated-p): Return animation delay in seconds.
Avoid bit manipulation in Lisp; use `delay' entry in the metadata.
(image-animate-timeout): Remove DELAY argument.  Use
image-animated-p to get animation delay for each frame.
(image-animate): Caller changed.

* src/image.c (gif_load): Add animation frame delay to the metadata.
(syms_of_image): Use DEFSYM.  New symbol `delay'.