Make DynamicImage non-exhaustive

[HeroicKatora]

As suggested by @5225225. Downstream code exhaustive matching on the current variants should use or petition for methods that extract into one types instead. Of particular note and example I'd like to point out [as_bytes] which essentially promises that all future variants will only contain matrices of trivial and padding-free channel types.

Overall, it's expected that this allows 0.24 to be more stable while we may discuss how to introduce half-float, or non-rgb combinations.

[fintelia]

Overall, it's expected that this allows 0.24 to be more stable while we may discuss how to introduce half-float, or non-rgb combinations.

And of course whether we should introduce half-float or non-rgb combinations 😄