Blender Git Loki

New undo cache management: Add Image IDs.

Some notes:
* `Image.cache` acts as some kind of 'main' cache, when it is NULL
(could not be restored), other caches should also be cleared. Oddly
enough, previous code was not clearing **all** caches, could not find
any reason for that behavior, so new code does a full clear.
* `imamap` is still used for Node previews from scenes' compositor,
however this is actually fully disabled in `direct_link_node()`.
* For render slots we cannot use offsetof as third part of the cache
key, so we are using a hash of the slot's name instead.

As far as I can tell, this fixes T76989: Visual glitches when undo after
reload multiple images by script (in Material Preview).