Map entities from a resource when written to the world.

[brandon-reinhart]

Objective

• Fix ReflectMapEntities on Resources #10958 by performing entity mapping on the entities inside of resources.

Solution

• Resources can reflect(MapEntitiesResource) and impl MapEntities to get access to the mapper during the world insert of the scene.

Testing

• A test resource_entity_map_maps_entities confirms the desired behavior.

Changelog

• Added reflect(MapEntitiesResource) for mapping entities on Resources in a DynamicScene.

fixes 10958

[alice-i-cecile]

Useful functionality, tightly scoped, with tests to validate that it works. Code quality is fine, you have my approval.

Like I said in the comment, I think we're out of luck for merging the two related types.

[alice-i-cecile]

I've added the S-Needs-Release-Note label here: I think that this is a high enough impact bug / limitation that it's worth calling out in a simple note so people can remove their workarounds.

[alice-i-cecile]

Thank you to everyone involved with the authoring or reviewing of this PR! This work is relatively important and needs release notes! Head over to bevyengine/bevy-website#1330 if you'd like to help out.