Blender Git Loki

LibOverride: Fix several issues with resync code.

This commit essentially touches to post-processing of collections and
objects after resync itself has been done, to ensure their proper
instantiation in the scene:

- Remove a lot of the process in resync case (resynced data are assumed
to be already instantiated in the scene, unlike override creation
case).

- For auto-resync, only do post-processing once after all overrides
have been resynced (doing it after each individual resynced was
causing a lot of instantiation glitches, with a lot of unwanted
extra objects and collections being added to the master collection).

It also deals in a much more reliable way with detection of objects
missing from the scene, by using the new `BKE_scene_objects_as_gset`
utils.

As a bonus this makes auto-resync process slightly faster (only by a few
percents, but that's always good to get).