VideoBackends: fix d3d12 subresource calculation

[iwubcode]

This pulls out the D3D12 fix for calculating the subresource from my post processing overhaul.

See section "subresource indexing" in https://learn.microsoft.com/en-us/windows/win32/direct3d12/subresources

Example, if our level count is 2 and our layer count is 3. We want to access the 2nd level and the 2nd layer.

We'd have this in master:

CalcSubresources(1, 1) => 4

vs this with the change:

CalcSubresources(1, 1) => 3

[Pokechu22]

The fix looks reasonable to me. Is this known to fix anything in practice on master? Also, is there any reason why D3D12CalcSubresource isn't in use? DXTexture.cpp (for D3D11) uses D3D11CalcSubresource.

[iwubcode]

Is this known to fix anything in practice on master

Not aware of anything, no. Then again, D3D12 is generally regarded as quite crashy. So who knows..

Also, is there any reason why D3D12CalcSubresource isn't in use? DXTexture.cpp (for D3D11) uses D3D11CalcSubresource.

Interesting, I wasn't aware of that function. Good question, not sure why that's not used. Maybe the helper was introduced to avoid the overhead of so many parameters, that's the only reason I can think of. Or maybe it just slipped the author's mind, this was in the original commit after-all.

I suppose I'll close this out. Someone can migrate to D3D12CalcSubresource instead (maybe I'll get to it at some point, gotta re-test with my PR though).

[iwubcode]

Ok, so decided to look at it more and see that D3D12CalcSubresource is not in the default header. It's in a helper header defined by MS: https://learn.microsoft.com/en-us/windows/win32/direct3d12/helper-functions-for-d3d12

Not sure if it's worth adding a whole header when just this function is fine. Maybe that was the original author's reasoning too. If we ever decide to introduce this header, we can revisit. Reopened for feedback.

[Pokechu22]

Ah, ok. We probably don't need to add the header. This implementation does match the implementation there (apart from not having planes), so it should be fine.