Blender Git Loki

Fix T77396: crash in memfile undo code after recent optimizations.

Optimizations in rBcda15408582e8de5b405 do not guarantee anymore that
consecutive memchunks in two consecutive undo steps are actually about
the same data (and hence can share the same buffer when unchanged).

This buffer sharing can now happen without any particular order, so we
need to change the process when 'merging' two undo memfiles together.

Note that existing code was not logically correct either, even with
previous undo storage code, since it would blindly transfer ownership of
the buffer to the second memchunk, without checking whether the first
one was actually the owner of it or not (a same buffer can be shared by
matching memchunks in many consecutive memfiles/undo steps).