Improve calculation of indent prefixes when using text scaling

Improve calculation of indent prefixes when using text scaling

Previously, the display spec for the "average space" was wrong.  It used
a plain number for the width of the specified space, which means "N
times the normal character width for the buffer", but we want "N times
the normal character width for the *current face*" (bug#76008).

* lisp/visual-wrap.el (visual-wrap--content-prefix): Specify the width
of the "average space" in terms of the average width of the current
face.

* lisp/net/shr.el (shr-indent): As above, and call 'string-pixel-width'
without specifying the buffer, to match 'shr-string-pixel-width'.

(cherry picked from commit 6842415577405adafcba16f8cbff02f35b5583f9)