Refactoring with bidi_fetch_char tested and debugged.

Refactoring with bidi_fetch_char tested and debugged.

 src/bidi.c (bidi_fetch_char): Accept also character position
 corresponding to BYTEPOS.  All callers changed.
 (bidi_cache_iterator_state, bidi_resolve_explicit_1)
 (bidi_resolve_explicit, bidi_resolve_weak)
 (bidi_level_of_next_char, bidi_move_to_visually_next): Abort if
 bidi_it->nchars is non-positive.
 (bidi_level_of_next_char): Don't try to lookup the cache for the
 next/previous character if nothing is cached there yet, or if we
 were just reseat()'ed to a new position.
 (bidi_paragraph_init, bidi_resolve_explicit_1)
 (bidi_level_of_next_char): Fix arguments in the calls to
 bidi_fetch_char.