Most of code additions are just gluing already implemented GstDxva and GstD3D12Decoder interfaces Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/5979>
Build gir for documentation Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/5481>
Extract Direct3D version independent common DXVA logic from d3d11 decoder so that it can be used by the other APIs (D3D9 and D3D12) as well Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/4161>