[opus] MS format tag for Opus codec

[Timothy B. Terriberry]

John Fredman wrote:
> Is there a defined format tag for Opus that should be used with the
> WAVEFORMATEX structure? See wFormatTag
> (http://msdn.microsoft.com/en-us/library/windows/desktop/dd390970.aspx).

There isn't one. Putting Opus into RIFF is complicated by the fact that 
it requires both variable-duration and variable-size frames, though I 
suppose something could be done along the lines that FFmpeg used for 
Vorbis (though how you would handle issues like pre-skip, end trimming, 
and pre-roll, I have no idea).

Why do you ask?