Apply `comment-depth' text properties when calling `back_comment'.

Apply `comment-depth' text properties when calling `back_comment'.

Change the definition of `comment-depth''s values from an atom to a cons so as
to hold enough information to initiliaze `scan_sexps_forward''s state.

* src/syntax.c (old_back_comment): renamed from `back_comment'.
(back_comment): New function which, when `comment-cacheing-flag' is non-nil
applies `comment-depth' text properties then (for now) calls
`old-back-comment'.
(scan_sexp_forward): Enhance values applied as `comment-depth''s values.