Actually fix the long lines display bug (bug#56393).

Actually fix the long lines display bug (bug#56393).

* src/dispextern.h (struct it): New 'narrowed_begv' field.

* src/dispextern.h (WITH_NARROWED_BEGV): New macro.

* src/xdisp.c (get_narrowed_begv): New function.
(init_iterator): Initilize the 'narrowed_begv' field.
(back_to_previous_line_start, get_visually_first_element,
move_it_vertically_backward): Use the new macro.

* src/dispextern.h: Prototype of 'get_narrowed_begv'.

* src/window.c (window_body_height): Make it externally visible.

* src/window.h: Prototype of 'window_body_height'.

* src/composite.c (find_automatic_composition): Optimize display in buffers
with very long lines with 'get_narrowed_begv'.

* lisp/obsolete/longlines.el: Reobsolete longlines-mode.

* etc/NEWS: Announce the new minor mode, and remove the unobsoletion
indication for 'longlines-mode'.

* doc/emacs/trouble.texi (Long Lines): Remove the section.
(Lossage): Remove the entry for the Long Lines section.

* doc/emacs/emacs.texi (Top): Remove the entry for the Long Lines section.