Re-enable Amber asset engine.

Update Amber to take into account new View level.

Also get rid of hack mixing repo and asset uuid into same UUID, now we
do have a real repo UUID in API.

Everything seems to be be back on par with pre-View changes status.

Fix various mistakes and forgotten bits in previous 'Views' adding commit.

Core API code shall now be mostly functional again.

Merge branch 'id_override_static' into asset-engine

Conflicts:
source/blender/makesdna/DNA_scene_types.h

Add an 'atomic cas' wrapper for pointers.

Avoids having to repeat obfuscating castings everywhere...

Merge branch 'master' into id_override_static

Tweak to fix for thread concurency in looptri generation.

Even if pointer assignment may be atomic, it does not prevent reordering
and other nifty compiler tricks, we need a memory barrier to ensure not
only that transferring pointer from wip array to final one is atomic,
but also that all previous writing to memory are ?flushed? to
(visible by) all CPUs...

Thanks @sergey for finding the potential (though quite unlikely) issue.

Fix T52875: install_deps.sh - numpy-1.13.1.tar.gz - 404 Not Found.

Asset API: add Views.

This commit add new 'View' forth layer, below history one (Revision).

It also adds a real UUID for the asset repository (as usual, asset
engines not needing it can just set it to zeros), will make Amber's life
(and all multi-repositories engines) easier.

Also fix one or two issues found on the road.

Code builds and links datablocks from libraries in the 'old way'. Real
test will come when re-enabling Amber and adapting it to new API though.

Disable for now asset engines (in preparation of core code changes).