Instead of relying on GScanner and its idea of syntax, code up a parser
that obeys the CSS spec.
This also has the great side effect of reporting correct line numbers
and positions.
Also included is a reorganization of the returned error values. Instead
of error values describing what type of syntax error was returned, the
code just returns SYNTAX_ERROR. Other messages exist for when actual
values don't work or when errors shouldn't be fatal due to backwards
compatibility.
