Fix use of ':align-to' in 'wrap-prefix'

Fix use of ':align-to' in 'wrap-prefix'

* src/dispextern.h (struct it): New flag 'align_visually_p'.
* src/xdisp.c (handle_line_prefix): Set the 'align_visually_p'
flag for 'wrap-prefix'.
(produce_stretch_glyph): If 'align_visually_p' flag is set, count
the :align-to offset from the beginning of the screen line, not
from BOL.  (Bug#71605)

* doc/lispref/display.texi (Truncation, Specified Space): Document
the special handling of ':align-to' in 'wrap-prefix'.