emacs/show: special case toggling display of images

According to emacs upstream [1], we can't expect overlay invisibility
and images to get along. This commit uses the previously stashed
undisplayer functions to actually remove the images from the buffer.
When the image is toggled, it is essentially redisplayed from scratch,
using the previously stashed redisplay data.

[1]: https://lists.gnu.org/archive/html/emacs-devel/2023-08/msg00593.html