Fix bytemuck feature after bitflags update

[nazar-pc]

Docs were not checked in CI, but failed somehow after merging to main: https://github.com/Rust-GPU/rust-gpu/actions/runs/30995586739/job/92271767273.

Copy/Clone derived unconditionally, despite only being needed for bytemuck. Feel free to update if that is not desirable (I think it is though).

[nazar-pc]

Will have a bit better version in a moment

[nazar-pc]

Okay, better solution is to simply enable bytemuck feature on bitflags itself. But consider adding docs checks to PRs as well so this can be caught earlier in the future.

[Firestar99]

I'm more concerned about this breaking change from bitflags v1 -> v2:

⚠️ Traits

Generated flags types now derive fewer traits. If you need to maintain backwards compatibility, you can derive the following yourself:

#[derive(PartialEq, Eq, PartialOrd, Ord, Hash, Debug, Clone, Copy)]

We should probably derive all that, I'll add it along with a check for the bytemuck feature.

[nazar-pc]

Makes sense