bug#23675: 30.0.50: make count-lines optionally ignore invisible lines

[Eli Zaretskii <eliz@gnu.org>]

> Cc: emacs-erc@gnu.org, rsw@gnu.org
> From: "J.P." <jp@neverwas.me>
> Date: Fri, 04 Aug 2023 17:35:27 -0700
> 
> Perhaps I'm not understanding how the `ignore-invisible-lines' parameter
> for `count-lines' is supposed to work, but it doesn't seem to make the
> function consider `invisible' text properties that have lists as values.
> I bring this up because ERC will likely be needing a line-counting
> function that's list-aware, at least in the manner shown in the attached
> tests. The change to `count-lines' accompanying these tests was merely
> lifted from `forward-visible-line' to make them pass, but it's quite
> possibly flawed and/or incomplete. If a proper solution ever emerges to
> address this, hopefully it'll come at the hands of someone better
> informed than I in the ways of Emacs invisibility. In the meantime
> (2023), ERC will likely be doing its own subpar rendition unless someone
> takes up the challenge for 30.1 (and Compat agrees to adopt it). Thanks.

This is OK for master, but please don't use cl-incf in simple.el, as
there's no real need to do so there.

Stefan, any comments?