A core meta which helps implement the old concept
of sub-buffering in some situations, by making it
possible for a buffer to keep a ref on a different
parent buffer. The parent buffer is unreffed when
the Meta is freed.

This meta is used to ensure that a buffer whose
memory is being shared to a child buffer isn't freed
and returned to a buffer pool until the memory
is.

https://bugzilla.gnome.org/show_bug.cgi?id=750039