To be used with the new d3d11 hwaccel decode API.

With the new hwaccel API, we don't want surfaces to depend on the
decoder (other than the required dimension and format). The old D3D11VA
pixfmt uses ID3D11VideoDecoderOutputView pointers, which include the
decoder configuration, and thus is incompatible with the new hwaccel
API. This patch introduces AV_PIX_FMT_D3D11, which uses ID3D11Texture2D
and an index. It's simpler and compatible with the new hwaccel API.

The introduced hwcontext supports only the new pixfmt.

Frame upload code untested.

Significantly based on work by Steve Lhomme <robux4@gmail.com>, but with
heavy changes/rewrites.

Merges Libav commit fff90422.
Signed-off-by: Diego Biurrun <diego@biurrun.de>