Fix vertical cursor motion when the newline is covered by a display string.

Fix vertical cursor motion when the newline is covered by a display string.

 src/indent.c (Fvertical_motion): Compute and apply the overshoot
 logic when moving up, not only when moving down.  Fix the
 confusing name and values of the it_overshoot_expected variable;
 logic changes accordingly.  (Bug#9254) (Bug#9549)
 src/xdisp.c (pos_visible_p): Produce correct pixel coordinates when
 CHARPOS is covered by a display string which includes newlines.
 (move_it_vertically_backward): Avoid inflooping when START_CHARPOS
 is covered by a display string with embedded newlines.