(beg_unchanged, end_unchanged, unchanged_modified,

(beg_unchanged, end_unchanged, unchanged_modified,
overlay_unchanged_modified): Removed.
(with_echo_area_unwind_data): Don't save beg/end_unchanged.
(unwind_with_echo_area_buffer): Don't restore them.
(debug_beg_unchanged, debug_end_unchanged) [GLYPH_DEBUG]: Removed.
(text_outside_line_unchanged_p, redisplay_internal,
try_scrolling): Use/set buffer-specific beg/end_unchanged.
(redisplay_window): Let try_window_id be called if more than one
window is displayed.  Use/set buffer-specific beg/end_unchanged.
(get_last_unchanged_at_beg_row, get_first_unchanged_at_end_row,
try_window_id):
Use buffer-specific beg/end_unchanged.
(beg_unchanged, end_unchanged, unchanged_modified,
overlay_unchanged_modified): Remove extern declarations.