rtp/rtpbuffer: Work on `&BufferRef` / `&mut BufferRef` instead of `&Buffer` / `&mut Buffer`
The buffer, in the writable case, needs to be actually writable (i.e. refcount 1) and this was previously not ensured.
Also it caused some API mismatch when trying to use it e.g. as part of
basetransform's transform_ip() virtual method.