Fix cursor motion at end of line that ends with a composite character.

Fix cursor motion at end of line that ends with a composite character.

 xdisp.c (display_line): Use `reseat' instead of `reseat_1', and
 use `get_next_display_element' and `set_iterator_to_next' to
 advance to the next character, when looking for the character that
 begins the next row.