Split Main world and Render world Entity space

[re0312]

Objective

• Another way to get retain render world
• Retain Rendering World #14449 proposes using an entity-to-entity mapping to avoid clearing all entities every frame in render world.However, it is confusing that main world entity may has a different entity ID to its corresponding entity in render world. It is foreseeable that it will significantly disrupt the existing rendering ecosystem and increase the maintenance burden on rendering personnel.

Solution

• The idea of this PR is similar to subnet division ,it should allow people to divide the entity space into multiple worlds, and the world default entity allocation will only be in its own entity space, unless you call get_or_spawn to share entities across worlds.
• In this PR, just simply allocate all even entity spaces to the main world and leave the rest to the rendering world.

Advantages

1. Almost no burden to rendering people and break change is minimal.
2. May open another door for assets as entities and networks
3. No need to occupy any bit space in the Entity

Drawbacks

1. Need to adjust the entity allocation method and the interface exposed to users.
2. Extra memory usage , WorldCount * (20 * EntityCount + ( SparseSetComponentCount * 8 * SparseSetEntity) bytes, for example , after this PR, the memory of many_cubes (2 worlds ,16w Entities) increased from 380 mb to 390mb
3. get(_mut) and SparseSet itertaion performance will decrease due to cache locality ( no more than 10%)

Performance

TODO

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Migration Guide

TODO

[bnyu]

Entity subspace, which looks like it may meets my needs.
I want my game main logic in lockstep to have deterministic, but others like UI or windows component entities are no need to have same in every player's games. I know this is not about render just say it, thank you for contributions.
So maybe it's better not to divide it with odd and even, maybe continuous blocks (may more cache-friendly), and there can be more defined block in the future?

[tychedelia]

Can we close this?

[atlv24]

Closed as Retained render world has been merged #15320