RTP buffer, mutable access to payload

Hey,

I'm trying to figure out how to write a payload to RTP buffer.

Consider the following code:

    let mut buf = gst::Buffer::new_rtp_with_sizes(42, 0, 0).unwrap();

    {
        let buf_mut = buf.get_mut().unwrap();
        let mut rtpbuf = RTPBuffer::from_buffer_writable(buf_mut).unwrap();

        let payload = rtpbuf.payload().unwrap();
        payload.copy_from_slice(&data[..]);
    }

Obviously, I cannot fill the payload because payload() returns Result<&[u8], ...>

I guess, I could fill the payload via gst::Buffer but I will need to know the actual offset of payload. It feels like, either I'm missing something or something is missing from API :) ?