Blender Git Loki

Render preview: Always make sure all ID datablocks references by objects are in bmain

Otherwise we'll have confused dependency graph builder, which wouldn't be able to
build proper graph.

Didn't find a way to avoid world copy here, we can probably escape with some shallow
copy here, but that will currently complicate code a lot.

Ideas to consider here:

- Use shallow copy of existing world after new ID management API is in place.

Downside would be thread safety, kind of nice to have everything local.

- Switch depsgraph away from ID_TAG and do hash lookup or so.

This will slow down depsgraph builder, but will make code more reliable.