optimize utf8 check tests into state bit

optimize utf8 check tests into state bit

Considering we go through it once per incoming char, the tests to see if we
should be checking utf-8 are too expensive... move them to a bit that lives
in the wsi and set them once per frame (except for CLOSE who has to update
after the close code has been skipped).

Signed-off-by: Andy Green <andy.green@linaro.org>