* lisp/font-lock.el (font-lock--syntax-table-affects-ppss): New var

* lisp/font-lock.el (font-lock--syntax-table-affects-ppss): New var

This tries to make `font-lock-syntax-table` work correctly even when
it changes the parsing of strings and comments, as was the case in
`font-latex.el`.

We should probably deprecate the use of `font-lock-syntax-table` since
the present fix is still not 100% and since it comes with performance
problems in large files.

(font-lock-set-defaults): Set it.
(font-lock-fontify-syntactically-region): Don't use `syntax-ppss`
when we think that `font-lock-syntax-table` would interfere.