[opus] RFC 8251 approved, now enabled by default

[Jean-Marc Valin]

Hi,

Just to let everyone know that RFC 8251, aka "Updates to the Opus Audio
Codec" has been approved by the IETF:
https://tools.ietf.org/html/rfc8251
This means that the two changes that were previously behind a
--enable-update-draft option are now enabled by default in git. If
needed (not sure why), they can still be disabled using --disable-rfc8251

The changes require a new set of testvectors, which are available at
https://www.opus-codec.org/docs/opus_testvectors-rfc8251.tar.gz

You can expect a 1.3-beta release in the near future, with a final 1.3
hopefully before the end of the year.

Cheers,

	Jean-Marc