Improve computation of indent depth in SHR and 'visual-wrap-prefix-mode'

Improve computation of indent depth in SHR and 'visual-wrap-prefix-mode'

Now, we get the average-width of the current font using
'string-pixel-width' and a specified space display spec, which doesn't
require the buffer to be displayed in a window (bug#72771).

* lisp/net/shr.el (shr-indent):
* lisp/visual-wrap.el (visual-wrap--content-prefix): Fix getting the
font when the buffer isn't displayed in a window.
(visual-wrap-fill-context-prefix): Fix indentation.

(cherry picked from commit 55aad592e177dc2c503ebe9ad2a46e227683315e)