Fix string handling to avoid data relocation gotcha.

Fix string handling to avoid data relocation gotcha.
Bugs in mode-line display still there.

 src/xdisp.c (compute_display_string_pos)
 (compute_display_string_end, reseat_to_string): Don't assume
 it->bidi_it.string.s always points to string.lstring's data.
 src/bidi.c (bidi_fetch_char, bidi_paragraph_init)
 (bidi_resolve_explicit_1, bidi_resolve_explicit)
 (bidi_resolve_weak, bidi_level_of_next_char): Don't assume
 string.s always points to string.lstring's data.