hwb parsing and rounding

[waywardmonkeys]

In the WPT, there's a test for parsing hwb(120 30% 50%) which says it should return rgb(77, 128, 77).

Instead, it returns rgb(76, 128, 76) using this code:

let check_color = parse_color(check).unwrap().to_alpha_color::<Srgb>().to_rgba8();
let result_color = parse_color(result).unwrap().to_alpha_color::<Srgb>().to_rgba8();
assert_eq!(check_color, result_color);

[raphlinus]

This is an unfortunate case in WPT, as the actual value is 76.5 127.5 76.5. It is likely caused by our use of multiplication by (1. / 255.). I don't think this is a reasonable thing to test, for example GPUs will likely get it wrong.