Start fixing cursor motion around invisible text.

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.