Fix the use case of popping from display property.
src/dispextern.h (struct iterator_stack_entry, struct it): New
member from_disp_prop_p.
src/xdisp.c (push_it, pop_it): Save and restore from_disp_prop_p.
(handle_single_display_spec, push_display_prop): Set the
from_disp_prop_p flag.
(get_overlay_strings_1): Reset the from_disp_prop_p flag.
(pop_it): Call iterate_out_of_display_property only if we are
popping after iteration over a string that came from a display
property. Fix a typo in popping stretch info. Add an assertion
for verifying that the iterator position is in sync with the bidi
iterator.
(handle_single_display_spec, get_overlay_strings_1)
(push_display_prop): Fix initialization of paragraph direction for
string when that of the parent object is not yet determined.