Add D3D12 pixel history implementation

[skarolewics]

It is disabled by default but can be enabled in the config editor via D3D12/PixelHistory.

This is the last commit needed for testing and continued validation/improvement of D3D12 pixel history. There will be one follow-up commit to add an automated smoke test (similar to the Vulkan pixel history test).

[Zorro666]

Thank you for all your work on this - it is a great starting place.

One minor change to make the config option be developer only until the feature is more tested and robust.

I gave it a quick trial on a simple D3D12 draw triangle capture I have and it worked to get the per fragment colour but didn't appear to be correct at a per event i.e. the clear result was the wrong colour and the one fragment that touched the pixel didn't get updated correctly but the per fragment value was correct.

I have seen this before in the other API pixel history : it usually a quick fix which will can be the starting point after this PR and the follow up test PR is merged.