[pixeldata] Feature gate capabilities: image, ndarray, rayon

[Enet4]

These changes make it so that image and ndarray support in the pixeldata crate become opt-in, and make it possible for projects to exclude the use of rayon entirely.

Since this depends on a very recent version of the compiler (1.60), I might postpone this to a later release. Even though the project currently has no compiler compatibility guarantees other than latest stable, I would like to prevent unnecessary surprises to those who do not update their compiler right away.