[opus] Suggest exposing VAD info. from opus frame

[강인규]

Hi.
I would like to contribute to the project regarding exposing VAD
information from opus frame.
I think VAD flag is very useful in jitter buffer management.
And it can be used at end-point detection for voice recognition, too.
As you know, there isn't VAD flag in CELT mode. So, in my proposal all
frames are assumed active signal in CELT mode.
It works on only single packet.
In multiple packet case, we can split it to single packet by repacketizer
in opus.
Please review.

Thanks,
InGyu Kang
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.xiph.org/pipermail/opus/attachments/20170905/3c29c538/attachment.html>
-------------- next part --------------
A non-text attachment was scrubbed...
Name: opus_vad_parser.c
Type: text/x-csrc
Size: 1219 bytes
Desc: not available
URL: <http://lists.xiph.org/pipermail/opus/attachments/20170905/3c29c538/attachment.c>
-------------- next part --------------
A non-text attachment was scrubbed...
Name: opus_vad_parser.h
Type: text/x-chdr
Size: 1226 bytes
Desc: not available
URL: <http://lists.xiph.org/pipermail/opus/attachments/20170905/3c29c538/attachment.h>