Why MarshalToSizedBuffer is in reverse order when use the latest gogofaster

[bestans]

Now, I cannot Marshal two message into one fixed size buffer unless re-allocated buffer.

func (m *MessageHead) XXX_Marshal(b []byte, deterministic bool) ([]byte, error) {
  if deterministic {
    return xxx_messageInfo_MessageHead.Marshal(b, m, deterministic)
  } else {
    b = b[:cap(b)]   // Notice! MarshalToSizedBuffer will fill data into b from the end
    n, err := m.MarshalToSizedBuffer(b)
    if err != nil { 
      return nil, err
    }
    return b[:n], nil
  }
}