Speed up redisplay of binary files with long series of nulls

Speed up redisplay of binary files with long series of nulls

* src/bidi.c (bidi_resolve_weak): Avoid entering a loop searching
for a character needed for resolving the type of a series of BN
and ET characters, as required by rule W5 of UAX#9, if the results
of the resolution are known in advance, because we are at level
zero, and the previous strong character was L.
(bidi_resolve_neutral): Partially resurrect the optimization for a
long series of control characters in an otherwise strictly L2R
text.
(bidi_level_of_next_char): Don't enter the loop that searches for
a paragraph separator if the current character is already at base
embedding level.  (Bug#22739)