Fix the different image zoom levels in SHR to work as expected

Fix the different image zoom levels in SHR to work as expected

* lisp/net/shr.el (shr-image-zoom-levels): New option.
(shr-image-zoom-level-alist): New variable.
(shr-zoom-image): Take POSITION and ZOOM-LEVEL arguments.  Consult
'shr-image-zoom-levels'.
(shr-put-image): Use 'shr-image-zoom-level-alist'.
(shr-rescale-image): Only reset width *or* height when either is too
large.
(shr--image-zoom-original-size, shr--image-zoom-image-size)
(shr--image-zoom-fill-height): New functions.

* etc/NEWS: Announce this change.

(cherry picked from commit 208207c1c07fb4669c6b7d64c27236074f996ae4)