(C) 1996 AROS - The Amiga Replacement OS
If the stream was of PushChunked() as IFFSIZE_UNKNOWN, we have to seek backwards and write the correct size. Since non RSEEK streams are automagically buffered by WriteStream, we don't have to fiddle with it here. However if the stream is buffered, we MUST see if we are about to pop the chunk that started the Buffering (in PushChunk)