lavu: Add a pixel format for new VDA hwaccel

The current hwaccel is broken and cannot be fixed in a compatible
way. It will be deprecated and replaced with a new one.
Merge commit '66e6c8a7b4e3ac926302529606bd3e323c28b20f'

* commit '66e6c8a7b4e3ac926302529606bd3e323c28b20f':
  lavu: Add a pixel format for new VDA hwaccel

Conflicts:
	doc/APIchanges
	libavutil/pixfmt.h
	libavutil/version.h

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