lavc: Add new VDA hwaccel

It leverages the new hwaccel 1.2 features:

- get_buffer2 is never called
- the internal context is automatically initialized/deinitialized

Signed-off-by: Luca Barbato <lu_zero@gentoo.org>
Merge commit '67afcefb35932b420998f6f3fda46c7c85848a3f'

* commit '67afcefb35932b420998f6f3fda46c7c85848a3f':
  lavc: Add new VDA hwaccel

Conflicts:
	configure
	libavcodec/vda.h
	libavcodec/vda_h264.c

Merged-by: Michael Niedermayer <michaelni@gmx.at>
avcodec/vda_h264: fix null pointer dereference

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