Basically copied from VLC (LGPL):

http://git.videolan.org/?p=vlc.git;a=blob;f=modules/video_output/win32/direct3d11.c;h=e9fcb83dcabfe778f26e63d19f218caf06a7c3ae;hb=HEAD#l1482
http://git.videolan.org/?p=vlc.git;a=blob;f=modules/codec/avcodec/d3d11va.c;h=85e7d25caebc059a9770da2ef4bb8fe90816d76d;hb=HEAD#l599

Merges Libav commit cfc9e7c9.
Signed-off-by: Luca Barbato <lu_zero@gentoo.org>

Some devices (some phones, apparently) will support only this opaque
format. Of course this won't work with CLI, because copying data
directly is not supported.

Automatic frame allocation (setting AVCodecContext.hw_device_ctx) does
not support this mode, even if it's the only supported mode. But since
opaque surfaces are generally less useful, that's probably ok.

Merges Libav commit 5030e385.
Signed-off-by: Luca Barbato <lu_zero@gentoo.org>

Makes dealing with formats that can not be used for staging textures
easier (DXGI_FORMAT_420_OPAQUE). It also saves memory if the staging
texture is never needed, so this is a good thing.

Merges Libav commit 98d73e41.
Signed-off-by: Luca Barbato <lu_zero@gentoo.org>

It appears in this case, frames_ininit is called twice (once by
av_hwframe_ctx_init(), and again by unreffing the frames ctx ref).

Merges Libav commit 086321c6.
Signed-off-by: Luca Barbato <lu_zero@gentoo.org>

When targeting the UWP API subset, the LoadLibrary function is not
available (and the fallback, LoadPackagedLibrary, can't be used to
load system DLLs). In these cases, link directly to the functions
in the DLLs instead of trying to load them dynamically at runtime.

Merges Libav commit fd1ffa1f.
Signed-off-by: Martin Storsjö <martin@martin.st>

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>