Without this change the math in `CSSHWB::to_color()` is lacking some
precision to generate the correct value to hand to `Color::from_hsv()`.
More precisely, when converting `hwb(120 20 30)`, the HSV's value would
be calculated as `1 - .3`. However, it turns out that `1 - .3f != .7f`
and `1 - .3f` gives bad results down the road in `Color::from_hsv()`.
This example actually only requires `resolve_with_reference_value()` to
return a double. I changed the two others for symmetry.