Speed up display of RTL text with many character compositions

Speed up display of RTL text with many character compositions

* src/bidi.c (bidi_level_start): New function.
* src/dispextern.h (bidi_level_start): Add prototype.
* src/xdisp.c (compute_stop_pos, set_iterator_to_next)
(get_visually_first_element, next_element_from_buffer): Call
'bidi_level_start' when looking for composed characters backwards,
to set limit of searching back, instead of looking all the way to
BOB.  (Bug#69385)

(cherry picked from commit fe24a8c3c091c1e051fe6a8c1ec4fd30ca052ca7)