This also adds support to avconv (which is trivial due to the new
hwaccel API being generic enough).
The new decoder setup code in dxva2.c is significantly based on work by
Steve Lhomme <robux4@gmail.com>, but with heavy changes/rewrites.
Signed-off-by: Diego Biurrun <diego@biurrun.de>
This only supports one device globally, but more can be used by
passing them with input streams in hw_frames_ctx or by deriving new
devices inside a filter graph with hwmap.