Thinks whole bbox code needs a complete rewrite, one can see a lot of
old history in it, it has way too many functions doing
nearly-the-same-thing(c), it spreads in very inconsistent ways across a
lot of files, ... But have no time for this right now, and would not be
a good idea with Beta comming up close anyway.
So for now going the simple and (hopefully) sane & safe way: forbid
object-level functions to affect data-level bbox. Mesh and curve ones
would generate bbox in obdata instead of object, for some reason (all
other obdata types only use object's bbox ever). That may have been
working in old ages, but with CoW and threaded depsgraph this is just
calling for piles of issues.
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