Uses similar way of storing temp data as object copy paste, just
uses different read entrypoint which does not modify current bmain.
This gives ability to easily copy-paste poses from one blender to
another one.
Hopefully doesn't introduce user-measurable differences.
Request from Peer here in the studio.
Reviewers: mont29
Reviewed By: mont29
Subscribers: hjalti, fsiddi
Differential Revision: https://developer.blender.org/D2229
