Add support for animated webp images

Add support for animated webp images

* configure.ac (HAVE_RSVG): Also include the webpdemux library.
It was new in version 0.4.4, and we require 0.6.0, so it should be
safe.
* src/image.c: Include demux.h.
(enum webp_keyword_index, webp_format): Include :index for
animations.
(init_webp_functions): Add Windows LOAD_DLLs.
(struct webp_cache, webp_create_cache)
(webp_prune_animation_cache, webp_get_animation_cache): New
functions.
(webp_load): Support animated webp images (bug#54242).