lavc: add a VP9 decoder

Originally written by Ronald S. Bultje <rsbultje@gmail.com> with the
help of Clément Bœsch <ubitux@gmail.com>.

Further contributions by:
Anton Khirnov <anton@khirnov.net>
Luca Barbato <lu_zero@gentoo.org>

Signed-off-by: Luca Barbato <lu_zero@gentoo.org>
Signed-off-by: Anton Khirnov <anton@khirnov.net>