lavc: add a native Opus decoder.

Initial implementation by Andrew D'Addesio <modchipv12@gmail.com> during
GSoC 2012.

Completion by Anton Khirnov <anton@khirnov.net>, sponsored by the
Mozilla Corporation.

Further contributions by:
Christophe Gisquet <christophe.gisquet@gmail.com>
Janne Grunau <janne-libav@jannau.net>
Luca Barbato <lu_zero@gentoo.org>
Merge commit 'b70d7a4ac72d23f3448f3b08b770fdf5f57de222'

* commit 'b70d7a4ac72d23f3448f3b08b770fdf5f57de222':
  lavc: add a native Opus decoder.

Conflicts:
	Changelog
	configure
	libavcodec/version.h

Fate tests pass with both avresample as well as swresample based opus decoder, but
are disabled (reference files are very large so i want to think a day or 2 about
if theres an alternative or if they could be avoided, they also dont match the
official samples)

Merged-by: Michael Niedermayer <michaelni@gmx.at>
swresample: swr_close()

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
avcodec/opusdec: switch to swresample

This also fixes linking failures in doc/examples which where apparently
caused by the linking order between avcodec and avresample

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>