[dx12] add VS solution ReleaseDX12Agility

[yuxiaomao]

Add DX12 Agility SDK as a VS project solution for better DX12 feature support, see also

• Documentation: https://devblogs.microsoft.com/directx/gettingstarted-dx12agility/
• Download dll https://www.nuget.org/api/v2/package/Microsoft.Direct3D.D3D12/1.618.5 (rename to zip, dlls are in build/native/bin/x64)

Usage:

• Build hl.exe (ReleaseDX12Agility hl)
• Put D3D12Core.dll in ./D3D12/ folder (relative to hl.exe), the version is defined by HL_DX12_AGILITY_VERSION=618. It's also required when distributing the game.

Ref @TothBenoit