Improve the speedup of bidi display introduced in 2011-10-18T16:56:09Z!eliz@gnu.org...

Improve the speedup of bidi display introduced in 2011-10-18T16:56:09Z!eliz@gnu.org for bug#9771.

 src/dispextern.h (struct bidi_it): New member next_en_type.
 src/bidi.c (bidi_line_init): Initialize the next_en_type member.
 (bidi_resolve_explicit_1): When next_en_pos is valid for the
 current character, check also for next_en_type being WEAK_EN.
 (bidi_resolve_weak): Don't enter the expensive loop if the current
 position is before next_en_pos.  Record the bidi type of the first
 non-ET, non-BN character we find, in addition to its position.
 (bidi_level_of_next_char): Invalidate next_en_type when
 next_en_pos is over-stepped.