VideoBackends: Use DXGI 1.6 and D3D11_4

[Xphalnos]

This allows D3D11 and D3D12 to use DXGI 1.6 and D3D11 to use its latest version (11.4)

[BhaaLseN]

I'm curious if this actually builds, since it is pure header changes. Does this need anything else, like a specific version of the Windows SDK? Well, builds worked, so that answers my question.

[iwubcode]

This file is using references to these headers, so the headers need to stay. We adopt an "include what you use" mentality, you can't rely on the fact this is pulling in D3DBase.h because that file could hypothetically be rewritten in the future to not include those headers.

Please fix all files that had removed header references but actually need them.

[Xphalnos]

I did what you asked.
I left #include <dxgi.h> removed in D3DSwapChain.h and in D3D12SwapChain.h because it is not used.