Improvement for long lines in buffers with font locking.

Improvement for long lines in buffers with font locking.

* src/xdisp.c (get_narrowed_width): Factored out from 'get_narrowed_len'.
(get_narrowed_len): Updated to use 'get_narrowed_width'.
(get_closer_narrowed_begv): New function.
(SET_WITH_NARROWED_BEGV): Add parameter to the macro.
(back_to_previous_line_start): Use the new function.
(get_visually_first_element, move_it_vertically_backward): Update the
calls to the macro.

* src/dispextern.h: Prototype of 'get_closer_narrowed_begv'.
Fix the prototypes of 'get_narrowed_begv' and 'get_narrowed_zv'.