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.