Fix display of R2L strings in mode line.

Fix display of R2L strings in mode line.
Composed characters still don't work.

 src/xdisp.c (init_iterator): Don't initialize it->bidi_p for strings
 here.
 (reseat_to_string): Initialize it->bidi_p for strings here.
 (next_element_from_string, next_element_from_c_string)
 (next_element_from_buffer): Add xassert's for correspondence
 between IT's object being iterated and it->bidi_it.string
 structure.
 src/bidi.c (bidi_level_of_next_char): Fix the logic for looking up
 the sentinel state in the cache.