The issue was that the data-blocks of two different undo steps were detected to
be identical, even if the attributes changed. That's because even if the
implicitly-shared data was different, they were turned into the same pointer by
cadb3fe5c5 on write.
This patch makes it so that for undo steps, implicitly shared data does not use
the pointer stability feature (in a sense, implicit-sharing itself provides
pointer stability for undo steps already).
The main tricky aspect is that we need to know if a pointer is implicitly shared
in `writestruct_at_address_nr` and oftentimes that's called before the
corresponding shared data is actually written with `BLO_write_shared`. The
solution is to enforce that the blend-write code has to know what pointers are
implicitly-shared before they are written the first time. The simplest way to
ensure that is to call `BLO_write_shared` first. However, that's not always
possible, especially when the pointer is directly embedded in an ID. Therefore,
there is a new `BLO_write_shared_tag` function that can be used in such cases.
The undo performance for the file in #141262 is still fixed with this change.
Pull Request: https://projects.blender.org/blender/blender/pulls/148144
7ed85bfe17