Adapt compute_display_string_pos to iteration over strings.

Adapt compute_display_string_pos to iteration over strings.
Just compiled, not yet tested.

 src/xdisp.c (compute_display_string_pos): First arg is now struct
 `text_pos *'; all callers changed.  Support display properties on
 Lisp strings.
 (compute_display_string_end): Support display properties on Lisp
 strings.
 (init_iterator, reseat_1, reseat_to_string): Initialize the
 string.bufpos member to 0 (zero, for compatibility with IT_CHARPOS
 when iterating on a string not from display properties).
 src/bidi.c (bidi_fetch_char): Support strings with display
 properties.
 src/dispextern.h (struct bidi_string_data): New member bufpos.
 (compute_display_string_pos): Update prototype.