Fix automatic hscrolling of only the current line

Fix automatic hscrolling of only the current line

* src/xdisp.c (display_line): When hscrolling only the current
line, increment iterator's first_visible_x and last_visible_x
values to account for the hscroll.  This propagates the hscroll
effect on the iterator geometry all the way down to the
subroutines called by display_line, and avoids scrolling bugs
under large hscroll values.  (Bug#26994)