Add with_alpha and multiply_alpha on DynamicColor

[waywardmonkeys]

If the alpha channel is missing, these operations will do nothing.

Additionally, mark some functions on Missing as const.

[waywardmonkeys]

For DynamicColor::multiply_alpha, I think that if the alpha is missing (and therefore 0.0), then the alpha remains missing.

But the interaction between that and DynamicColor::premultiply_split is not clear to me, since it treats a missing alpha as being opaque:

    fn premultiply_split(self) -> ([f32; 3], f32) {
        // Reference: §12.3 of Color 4 spec
        let (opaque, alpha) = split_alpha(self.components);
        let premul = if alpha == 1.0 || self.missing.contains(3) {
            opaque
        } else {
            self.cs.layout().scale(opaque, alpha)
        };
        (premul, alpha)
    }

So for the purposes of multiply_alpha, would we set it to not be missing and then set the alpha to the factor?

Same question for with_alpha where we're setting the alpha. Would we leave it missing or set it and clear the missing flag?

There's no code that clears missing flags, so maybe I'm wrong here and we should document that both things effectively do nothing when the alpha is explicitly missing?

[waywardmonkeys]

We need multiply_alpha for supporting using DynamicColor with peniko::ColorStop / peniko::Gradient.

[waywardmonkeys]

I've answered my own question for now by leaving the color unchanged when modifying the alpha channel if the alpha channel is missing.

This is what would have happened if one had done the same operation via DynamicColor::map.

[raphlinus]

Looks good, thanks!