* lisp/whitespace.el: Use font-lock-flush. Minimize refontifications.

* lisp/whitespace.el: Use font-lock-flush.  Minimize refontifications.
Side benefit: it works without jit-lock.
(whitespace-point--used): New buffer-local var.
(whitespace-color-on): Initialize it and flush it.  Use font-lock-flush.
(whitespace-color-off): Use font-lock-flush.
(whitespace-point--used, whitespace-point--flush-used): New functions.
(whitespace-trailing-regexp, whitespace-empty-at-bob-regexp)
(whitespace-empty-at-eob-regexp): Use them.
(whitespace-post-command-hook): Rewrite.