Add approx

[ncihnegn]

To allow approx comparison of colors.

[kornelski]

Thanks for the PR. I'm not sure if this belongs to this crate, because:

• It adds a dependency. I only have bytemuck and serde, because they are required for interoperability with the crates ecosystem. I don't think approx is as necessary.

• Colors are usually compared using perceptual metrics, but these vary in complexity and depend on specifics of color spaces. Comparison of colors with alpha channel is even more complex, so there isn't a one good answer for everyone.