Blender Git Loki

UI/Assets: Support generating object preview images

Object previews are really helpful for visual data-block selection, like asset
browsing. Having them be generative should also be quite handy and should work
well enough in many, if not most cases.

What this does is simple:
* Place the object (actually a deep copy of it, for thread safety) in a virtual
.blend into an empty scene/view-layer.
* Add a camera, point it towards the front of the object, assuming that means
pointing towards its +Y axis.
* Use "Camera Fit Frame to Selected" logic to put the object into frame.
* Create a threaded off-screen render.

Of course, such an automatic preview will not work in all situations. E.g. it
currently does a bad job capturing a single plane. We could add options for
more advanced automatic previews, but probably custom previews is more
important, which I committed already (812ea9184221).

Part of the first Asset Browser milestone. Check the #asset_browser_milestone_1
project milestone on developer.blender.org.

Reviewed as part of https://developer.blender.org/D9719.
Reviewed by: Bastien Montagne, Brecht Van Lommel