meta: Set the parent refcount of the GstStructure correctly

meta: Set the parent refcount of the GstStructure correctly

The parent refcount is of the *transformed* buffer, not the input
buffer.

Also update the docs to clarify that @transbuf is the transformed
buffer, and not the buffer on which a transformation is being
performed.

Due to this bug, modifying the structure of a meta that has been
copied to another buffer fails with:

gst_structure_set: assertion 'IS_MUTABLE (structure) || field == NULL' failed

Add a test for the same.

Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/2890>