Objective

#15320 is a particularly painful breaking change, and the new RenderEntity in particular is very noisy, with a lot of let entity = entity.id() spam.

Solution

Implement WorldQuery, QueryData and ReadOnlyQueryData for RenderEntity and WorldEntity.

These work the same as the Entity impls from a user-facing perspective: they simply return an owned (copied) Entity identifier. This dramatically reduces noise and eases migration.

Under the hood, these impls defer to the implementations for &T for everything other than the "call .id() for the user" bit, as they involve read-only access to component data. Doing it this way (as opposed to implementing a custom fetch, as tried in the first commit) dramatically reduces the maintenance risk of complex unsafe code outside of bevy_ecs.

To make this easier (and encourage users to do this themselves!), I've made ReadFetch and WriteFetch slightly more public: they're no longer doc(hidden). This is a good change, since trying to vendor the logic is much worse than just deferring to the existing tested impls.

Testing

I've run a handful of rendering examples (breakout, alien_cake_addict, auto_exposure, fog_volumes, box_shadow) and nothing broke.

Follow-up

We should lint for the uses of &RenderEntity and &MainEntity in queries: this is just less nice for no reason.