Fix bug #15090 with redisplay under linum-mode and visual-line-mode.

Fix bug #15090 with redisplay under linum-mode and visual-line-mode.

 src/xdisp.c (compute_window_start_on_continuation_line): When
 WORD_WRAP is in effect, use move_it_to instead of move_it_by_lines
 to make sure we end up setting the window start at the leftmost
 visible character of the display line.  This avoids funky
 horizontal shifting because the window start is not kept on the
 same position.