Fix the decision about box_face in mode line for reordered text.

Fix the decision about box_face in mode line for reordered text.
This fixes display of composed characters in buffer name on mode line.

 src/xdisp.c (face_before_or_after_it_pos): Support bidi iteration
 when BEFORE_P is zero.
 (next_element_from_c_string): Handle the case of the first string
 character that is not the first one in the visual order.