Add support for variable-pitch fonts in 'visual-wrap-prefix-mode'

* lisp/emacs-lisp/subr-x.el (string-pixel-width): Allow passing BUFFER
to use the face remappings from that buffer when calculating the width.

* lisp/visual-wrap.el (visual-wrap--prefix): Rename to...
(visual-wrap--adjust-prefix): ... this, and support PREFIX as a number.
(visual-wrap-fill-context-prefix): Make obsolete in favor of...
(visual-wrap--content-prefix): ... this.
(visual-wrap-prefix-function): Extract inside of loop into...
(visual-wrap--apply-to-line): ... this.

* doc/lispref/display.texi (Size of Displayed Text): Update
documentation for 'string-pixel-width'.

* etc/NEWS: Announce this change.

1 files changed, 4 insertions, 2 deletions

diff --git a/doc/lispref/display.texi b/doc/lispref/display.texi
index 195464ef7f5..d28ff9ead26 100644
--- a/doc/lispref/display.texi
+++ b/doc/lispref/display.texi
@@ -2385,9 +2385,11 @@ The optional arguments @var{x-limit} and @var{y-limit} have the same
 meaning as with @code{window-text-pixel-size}.
 @end defun
 
-@defun string-pixel-width string
+@defun string-pixel-width string &optional buffer
 This is a convenience function that uses @code{window-text-pixel-size}
-to compute the width of @var{string} (in pixels).
+to compute the width of @var{string} (in pixels).  If @var{buffer} is
+non-@code{nil}, use any face remappings (@pxref{Face Remapping}) from
+that buffer when computing the width of @var{string}.
 @end defun
 
 @defun line-pixel-height