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.

Merges Libav commit f9e7a2f9.
Also adds untested VP9 support.
The check for DXVA2 COBJs is removed. Just update your MinGW to
something newer than a 5 year old release.
Signed-off-by: Diego Biurrun <diego@biurrun.de>