[opus] Opus in WebM

[Timothy B. Terriberry]

Brendan Bolles wrote:
> DiscardPadding seems to assume that you might top off an opus
> frame with empty samples when the frame would spill onto the next
> matroska timestamp.  So rather than have continuous opus that
> could be decoded one after another, you'd have to jump over these
> little gaps?

The intent was not to allow gaps in the file like this (that would 
confuse the encoder, compress badly, and complicate decoders, plus be 
unrepresentable in Ogg). The intent was to allow trimming from the 
_last_ frame in a stream to allow sample-accurate total duration. If 
there's some way to say that it cannot be used anywhere else in the 
spec, we should do so.

> The document says it would be useful for Vorbis too, but Vorbis
> seems to just emit packets as it wants and you don't really have
> any control over the boundaries.  I assumed we would just do the
> same with Opus, even it it can be controlled better.

Like Opus, the idea was to use it in the last frame only, for sample 
accurate duration. Right now if you mux a Vorbis file into MKV and then 
back into Ogg, the duration changes.