d3d12: Attempt to use the Agility SDK

d3d12: Attempt to use the Agility SDK

The ideal would be to use a D3D12Core.dll next to the GL DLL, via
ID3D12SDKConfiguration1, but this requires a new D3D12.dll to be
present on the machine, which only very new OSes will currently have.
For older OSes, we can at least try to use ID3D12SDKConfiguration to
set a global Agility SDK override, but that only works in developer
mode and requires the DLL to be at a relative path from the app EXE,
so only try that if we have environment variables telling us to do so.

Reviewed-by: Sil Vilerino <sivileri@microsoft.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/18305>