[D3D12] Dynamically load Agility SDK.

[bruvzg]

Instead of relaying on exported symbols and D3D12 loader, load SDK directly via device factory. This should allow using user specified Agility SDK version instead of hard-coded one, and fallback to the system D3D driver if it's not found (so we can make official builds with Agility support, but avoid shipping proprietary DLL that require uses to accept EULA).

Adds rendering/rendering_device/d3d12/agility_sdk_version property to control used SDK version.

Based on Mesa Agility loading code - https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/18305/diffs#b974333948460f1be995f150bf4db414b591b1b1

• Production edit: This partially addresses Add Direct3D 12 support to official Windows binaries #86490.

[RandomShaper]

A nice enhancement.

[akien-mga]

Thanks!

[Calinou]

It looks like this caused a build-time regression: https://forum.godotengine.org/t/error-while-compiling-godot-for-d3d12-with-scons/41654

[RandomShaper]

I've just found this approach won't work on an up-to-date Windows 10, such as mine:

The SDK config interface is not available until a later release (Windows 10 insider build or Windows 11, I guess), as said in https://learn.microsoft.com/en-us/windows/win32/api/d3d12/nn-d3d12-id3d12sdkconfiguration:

[bruvzg]

The SDK config interface is not available until a later release (Windows 10 insider build or Windows 11, I guess)

Since it loaded dynamically, I assume it won't break DX12, but only prevent it from loading Agility libs. I guess we should keep both predefined exports and factory approach in this case (will test if it works later).