Start working on invisible text. For now, just untested source changes.

Start working on invisible text.  For now, just untested source changes.

 xdisp.c (handle_invisible_prop): Under bidi iteration, skip
 invisible text using bidi_get_next_char_visually.
 (handle_stop_backwards): Restore it->bidi_p, it->current and
 it->position before calling handle_stop.
 (next_element_from_buffer): Fix formatting.