In SHR, keep track of image sizes as specified by the HTML

In SHR, keep track of image sizes as specified by the HTML

Previously, these values got lost when zooming the image.

* lisp/net/shr.el (shr-tag-img): Set 'image-dom-size'...
(shr-zoom-image): ... use it.  Rename 'size' to 'zoom'.
(shr-image-fetched): Rename 'image-size' to 'image-zoom'.
(shr-put-image): Accept the zoom level as ':zoom' and document it.
Previously, FLAGS was a mix of alist and plist(!).

* test/lisp/net/shr-tests.el (shr-test/zoom-image): Rename "size" to
"zoom".

(cherry picked from commit 6d082f3c79269f00308d6e8b7d31d6a119376fe2)