Allows to use regular constructor/destructors. And standard CPP data
like string, maps, etc.
Main is defined as non-movable and non-copyable for the time being.
The existing creating/deleting API is kept as-is, moving to direct
MEM_new/MEM_delete usages in the code is kept as a later cleanup commit.
The 'ListBase' of split-by-libraries Mains has been replaced by a VectorSet.
This is an unfortunate consequence of making Main a non-trivial C++
struct: it is no more guaranteed that next/prev pointers required for
ListBase remain immediately at the top of the struct anymore - and on
Windows they indeed don't.
So move to a more modern form of storage for split-by-libraries mains,
with a few other related changes to internal readfile code.
WARNING: This is quite a sensitive change, extensively tested locally
(including library-related changes across undos and redos, in complex
production scenes).
Pull Request: https://projects.blender.org/blender/blender/pulls/141086
841b518bf6