[opus] Opus in WebM

[Timothy B. Terriberry]

Brendan Bolles wrote:
> Whoops, I meant for the total duration of the file you should be
> able to have your vorbis length match the file duration length exactly.

A Vorbis stream allows only two frame sizes: short and long (how long 
those two are exactly depends on the encoding parameters, but the 
encoder must choose two). Both are always powers of 2. That's not enough 
to represent arbitrary durations.