Fix slow operation of 'string-width'

Fix slow operation of 'string-width'

* src/composite.c (find_automatic_composition): Accept one
additional argument BACKLIM; don't look back in buffer or string
farther than that.  Add an assertion for BACKLIM.
(composition_adjust_point, Ffind_composition_internal): Callers
adjusted.
* src/composite.h (find_automatic_composition): Adjust prototype.
* src/character.c (lisp_string_width): Call
'find_automatic_composition' with the value of BACKLIM equal to POS,
to avoid costly and unnecessary search back in the string, since
those previous characters were already checked for automatic
compositions.  (Bug#48734)  (Bug#48839)