bug#57207: 29.0.50; Fontification is slow after e7b5912b23 (Improvements to long lines handling)

[Gregory Heytings]

> Gregory, perhaps we should restrict the search even further, as this 
> loop could take some tangible time with very large files.  How about if 
> we search from window-start point instead of BEGV, and limit the search 
> by, say, 10,000,000 newlines and 200,000,000 characters, whichever 
> happens first?  Is it really likely to have files with so many short 
> lines, followed by long lines?

Yes, that will be part of the next iteration of the code.