Text covered by `display' overlays is correctly reordered.

Text covered by `display' overlays is correctly reordered.
Cursor positioning is not yet right near the overlay.

 src/xdisp.c (compute_display_string_pos): Non-trivial implementation.
 (compute_display_string_end): New function.
 src/dispextern.h (compute_display_string_end): Declare prototype.
 src/bidi.c (bidi_resolve_explicit_1): Use ZV for disp_pos.
 (bidi_fetch_char): Implement support for runs of characters
 covered by display strings.