d3d12: Add Direct3D12 H.264 video decoder

... including minimum set of required implementations.
Note that d3dx12.h header is officially maintained by MS
but it's not a part of Windows SDK. So application needs to
copy it by hands. See also
https://docs.microsoft.com/en-us/windows/win32/direct3d12/helper-structures-and-functions-for-d3d12

Another note is that since no other d3d12 element at the moment,
d3d12 decoder will output system memory by downloading every decoded
frame into system memory.
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