Support hscrolling only the current line

Support hscrolling only the current line

* src/xdisp.c (hscrolling_current_line_p): New function.
(init_iterator): If auto-hscrolling just the current line, don't
increment the iterator's first_visible_x and last_visible_x
variables.
(hscroll_window_tree): Recompute window's hscroll when moving
vertically to another screen line.
(redisplay_window): If we are hscrolling only the current line,
disable the optimizations that rely on the current matrix being
up-to-date.
(display_line): Accept an additional argument CURSOR_VPOS, the
vertical position of the current screen line which might need
hscrolling; all callers changed.  Compute first_visible_x and
last_visible_x specially when auto-hscrolling current line, by
repeating the calculation that is done in init_iterator in other
modes.
(syms_of_xdisp) <auto-hscroll-mode>: No longer boolean, it can now
accept a 3rd value 'current-line, to turn on the mode where
only the current line is hscrolled.

* etc/NEWS: Mention the new auto-hscroll-mode value.