Fix handling of 'min-width' display property

Fix handling of 'min-width' display property

* src/xdisp.c (get_display_property, display_min_width): Rename
BUFPOS to CHARPOS, to avoid confusion (it is not necessarily a
buffer position).  Suggested by Jim Porter <jporterbugs@gmail.com>.
(get_display_property): Call 'Fget_char_property' to support
'min-width' properties of overlays as well.
(display_min_width): Handle the buffer and string cases more
accurately, without relying only on the values of positions.
(handle_display_prop, handle_single_display_spec): Pass correct
position to 'display_min_width', when iterating over a string.
(handle_display_prop): When OBJECT is a window, pass it to
display_min_width.
(set_iterator_to_next): Call 'display_min_width' when at end of a
display or overlay string.  (Bug#72721)

* etc/NEWS: Announce the support for overlays.

(cherry picked from commit 71505b723c9fb9de20f6d38be7c73d595e9be3ce)