Adds supports for collection previews that are rendered automatically when
collections are marked as assets. (Or when preview rendering is triggered
differently, e.g. through the //Refresh Data-Block Previews// operator).
Idea in this patch is to create a collection instance empty outside of main for
the collection, and then reuse the object rendering code to render the preview.
This keeps things very simple and works just fine.
Differential Revision: https://developer.blender.org/D14460
Reviewed by: Bastien Montagne
810e225c26