This fixes a `FIXME` in `extract_meshes` and results in a performance
improvement.
As a result of this change, meshes in the render world might not be
attached to entities anymore. Therefore, the `entity` parameter to
`RenderCommand::render()` is now wrapped in an `Option`. Most
applications that use the render app's ECS can simply unwrap the
`Option`.
Note that for now sprites, gizmos, and UI elements still use the render
world as usual.
## Migration guide
* For efficiency reasons, some meshes in the render world may not have
corresponding `Entity` IDs anymore. As a result, the `entity` parameter
to `RenderCommand::render()` is now wrapped in an `Option`. Custom
rendering code may need to be updated to handle the case in which no
`Entity` exists for an object that is to be rendered.
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