Add code to initialize bidi iterator for displaying strings.

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.