bug#60953: The :match predicate with large regexp in tree-sitter font-lock seems inefficient

[Eli Zaretskii]

> Date: Thu, 2 Feb 2023 14:12:54 +0200
> Cc: casouri@gmail.com, 60953@debbugs.gnu.org
> From: Dmitry Gutov <dgutov@yandex.ru>
> 
> Please take a look at this additional patch on top of the other one. It 
> is ok?
> 
> I just remembered that if we don't want to auto-anchor the regexp, then 
> 'looking-at' is not the appropriate semantic. So this switches to 
> 'search_buffer'.

One gotcha: fast_looking_at returns the length of the match, whereas
search_buffer returns the buffer position where it matched.  Shouldn't
this need a different handling of the return value?