Fix display of :box face when overlay strings are around

Fix display of :box face when overlay strings are around

* src/xdisp.c (reset_box_start_end_flags): New function.
(handle_face_prop): Only set the start_of_box_run_p flag, don't
reset it.
(pop_it): Set the face_box_p flag, if the popped face requires
that, when continuing iteration over buffer text.
(get_next_display_element, next_element_from_display_vector): Only
set the end_of_box_run_p flag, never reset it here.
(set_iterator_to_next): Don't reset the start_of_box_run_p and
end_of_box_run_p flags here.  They are now reset as side effect of
PRODUCE_GLYPHS.
(append_space_for_newline): Restore the end_of_box_run_p flag
after PRODUCE_GLYPHS where we previously didn't reset it.
* src/dispextern.h (PRODUCE_GLYPHS): Call
reset_box_start_end_flags after producing glyphs.
(Bug#40124)