When data-block/asset previews were not stored in the thumbnail cache
yet (or were outdated), we'd read them from .blend files. This could
lead to random crashes (but quite reliable with a small number of
previews to be read).
Wasn't clearing runtime memory which could lead to the
`PRV_TAG_DEFFERED` bit being set. This meant we would try to free
deferred preview data since eefee47a8a, which was just garbage memory.
Previously, BKE level preview image code was in `BKE_icons.h` and `icons.hh`.
While these types are related, I always found this quite hard to navigate since
preview image stuff was just in the middle of icon functions. Plus, people
don't expect preview image functions in icon files, the relationship is not
obvious.
Instead, use focused files that make it easy to quickly navigate them
and see what they are dealing with.
Pull Request: https://projects.blender.org/blender/blender/pulls/111709
