[Vorbis] Stream "Saving" and Excerpting...

[Michael Smith]

> Put more generally, if you're about to insert headers that are
> identical to the previous set, you can just skip them, and append
> the new data, making sure the serial numbers match.

... as long as you ensure granulepos remains monotonically increasing
(modulo special values like -1).

Actually, you should probably take care to ensure that it's correctly
increasing (i.e. by the right amount, given the contents of the
pages), which is non-trivial to calculate.

Mike