buffer: drop parent meta in deep copy/foreach_metadata

buffer: drop parent meta in deep copy/foreach_metadata

The purpose of a deep buffer copy is to be able to release the source
buffer and all its dependencies. Attaching the parent buffer meta to
the newly created deep copy needlessly keeps holding a reference to the
parent buffer.

The issue this solves is the fact you need to allocate more
buffers, as you have free buffers being held for no reason. In the good
cases it will use more memory, in the bad case it will stall your
pipeline (since codecs often need a minimum number of buffers to
actually work).

Fixes #283

Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/2928>