Allow using variable-width fonts in eww

Allow using variable-width fonts in eww

* lisp/gnus/mm-decode.el (mm-shr): Only pass the fill column when not using
fonts, because limiting the width to what's appropriate for followups
doesn't really help when not using proportional fonts.

* lisp/net/shr.el (shr-use-fonts): New variable.
(shr-fill-text): Rename from "fold".
(shr-pixel-column, shr-pixel-region, shr-string-pixel-width): New
functions.
(shr-insert): Just insert, don't fill the text.  Filling is now
done afterwards per display unit.
(shr-fill-lines, shr-fill-line): New functions to fill text on a
per-unit base.
(shr-find-fill-point): Take a "beginning" parameter.
(shr-indent): Indent using the :width display parameter when using
fonts.
(shr-parse-style): Ignore "inherit" values, since we already do that.
(shr-tag-img): Remove the insertion states.
(shr-tag-blockquote): New-style filling.
(shr-tag-dd): Ditto.
(shr-tag-li): Ditto.
(shr-mark-fill): New function to mark lines that need filling.
(shr-tag-h1): Use a larger font.
(shr-tag-table-1): Get the natural and suggested widths in one
rendering.
(shr-tag-table): Create the "fixed" version of the table only once
so that we can cache data in the table.
(shr-insert-table): Get colspan calculations right by having
zero-width columns after colspan ones.
(shr-expand-alignments): New function to make :align-to specs work
right when rendered in one buffer and displayed in another one.
(shr-insert-table-ruler): Use :align-to to get the widths right.
(shr-make-table): Cache more.
(shr-make-table-1): Use the new <td> data layout.
(shr-pixel-buffer-width): New function.
(shr-render-td): Add a caching layer.
(shr-dom-max-natural-width): New function.