Enable GPU validation on DX12 and Vulkan

[kvark]

Is your feature request related to a problem? Please describe.
Quite often, DX12 can fail without any clear validation error. It would help a ton if we could get more information about those.

Describe the solution you'd like
See https://docs.microsoft.com/en-us/windows/win32/direct3d12/using-d3d12-debug-layer-gpu-based-validation
We'll need a separate instance flag for this, since the perf penalty is greater.

Describe alternatives you've considered
Shoving this behind the existing VALIDATION flag

Additional context
Similar change will be needed in Vulkan. Distantly related to #1708

Checklist

• Enable GPU-based validation for DX12 #5146
• Enable GPU-based validation for Vulkan #5046

[ErichDonGubler]

I've edited the OP to contain a checklist of PRs I've filed to resolve this issue.