Adds the necessary wiring to get VP9 to work with `ivfwriter`.
Update the README of save-to-disk to inform users it supports
both VP8 and VP9.
ivfwriter currently assumes 30 fps but it seems that the other codecs
also assume 30 fps so that is not a net-new assumption.
In Go 1.22 and earlier, a ticker needs to be explicitly stopped
when it's no longer useful in order to avoid a resource leak.
In Go 1.23 and later, an orphaned ticker will eventually be
garbage collected, but it's still more thrifty to stop it early.
