[opus] Extension Packets?

[William Swartzendruber]

I have a project I'm investigating.  The goal is to basically add lossless
extensions to Opus.  You have an Opus stream with standard packets, but
interwoven in there are extension packets that contain the residuals.
 Ideally, compliant decoders play the stream back and ignore the extension
packets.  This (hopefully) makes the "lossless" stream compatible with
existing players.  Specialized decoders decode the standard Opus (CELT)
packets, the residual extension packets, and then combines them to
reproduce a bit-exact stream to the original PCM.

Now those of you familiar with how Opus works will readily identify some
issues with why this might be completely impossible.  Those problems are to
be explored later.  First, I need to know that current players can even
handle streams with extension packets.  The problem is that, currently,
these packets are completely undefined.

My current idea is to have a packet with zero frames (illegal) and then put
the extension data in that (illegal) packet's padding area.  As I remember
it, this is against the Opus RFC for at least two reasons:

1. An Opus packet must have at least one frame.
2. A packet's padding may not contain anything but null bytes.

However, the spec also says packets that violate this should simply be
ignored, which is what I'm hoping players do.  Another question is how
streams with extension packets get muxed into containers like Matroska.
 Will mkvtools drop the extension packets?

So how should I go about structuring these things?  I assume somewhere in
the padding I need to place an identifier in there so that later decoders
don't confuse my extension packets with other types.
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