Fix WHITESPACE issues in the CSS grammar.
A single WHITESPACE token consumes consecutive spaces, but does not consume
spaces separated by comments. That means S* and S+ in CSS grammars need to
accept multiple WHITESPACE tokens. Additionally, white spaces are not
mandatory to separate an @-symbol and the rest of the prelude.
Use space non-terminal instead of WHITESPACE for S+ in calc expressions.
Use maybe_space non-terminal instead of WHITESPACE for S* after @-webkit-filter