Add dithering for From<RGBf> for RGBu8/RGBu16

[virtualritz]

I.e. what good old RiQuantize does. I think it's safe to assume a dither amplitude of 0.5.
This should be built-in. I do not see anyone wanting to see banding when they do a conversion to one of those two types.

Only challenge I see is a nice API for seeding the RNG so results are deterministic on re-runs.

Rust version of RiQuantize:

    use num_traits::float::Float;
    use core::ops::Mul;
    use oorandom;

    pub fn quantize<T>(value: T, one: T, min: T, max: T, dither_amplitude: T, rng: &mut oorandom::Rand32) -> T
    where
        T: Float + Mul<f32, Output = T>
    {
        use clamped::Clamp;
        (one * value + dither_amplitude * rng.rand_float())
            .round()
            .clamped(min, max)
    }