Start fixing cursor motion around invisible text.
Hit a MAJOR design problem in pushing and popping bidi iterator state.
src/xdisp.c (reseat_1): Call bidi_init_it to resync the bidi
iterator with IT's position.
(handle_stop, back_to_previous_visible_line_start, reseat_1):
Reset the from_disp_prop_p flag.
src/bidi.c (bidi_cache_search): Don't assume bidi_cache_last_idx is
always valid if bidi_cache_idx is valid.
(bidi_cache_find_level_change): xassert that bidi_cache_last_idx
is valid if it's going to be used.