Add code to initialize bidi iterator for displaying strings.
For now, ifdef'ed away. Some more testing.
src/bidi.c (bidi_level_of_next_char): Allow the sentinel "position"
to pass the test for valid cached positions.
src/xdisp.c (init_iterator): Call bidi_init_it only of a valid
buffer position was specified. Initialize paragraph_embedding to L2R.
(reseat_to_string): Initialize the bidi iterator (for now ifdef'ed out).
(display_string): If we need to ignore text properties of
LISP_STRING, set IT->stop_charpos to IT->end_charpos. (The
original value of -1 will not work with bidi.)
src/dispextern.h (struct bidi_string_data): New member lstring.