Use placeholder images in shr to avoid text moving around

Use placeholder images in shr to avoid text moving around

* lisp/net/shr.el (shr-rescale-image): Pass in width/height
from the HTML.
(shr-tag-img): Ditto.
(shr-string-number): New function.
(shr-make-placeholder-image): Make placeholder images.
(shr-tag-img): Insert them if we have SVG support.