[idd] isolate post-processing by framebuffer slot

Give each in-flight framebuffer a complete post-processing chain and
its own COMPUTE recording slot while retaining one physical queue.

Claim framebuffer ownership before submitting compute work, and use
the frame index for both COMPUTE and COPY recording state. Drain
in-flight work before reconfiguring either chain so COPY never
references replaced effect resources.

Require both chains to contain the same effects, preserve pending
damage, and release ownership on every pre-copy failure path.

idd/LGIdd/CD3D12Device.h

@@ -79,6 +79,6 @@ struct CD3D12Device
     ComPtr<ID3D12Heap   > GetHeap() { return m_ivshmemHeap; }
     bool IsIndirectCopy() { return m_indirectCopy; }
 
-    CD3D12CommandSlot * GetCopySlot(unsigned frameIndex);
-    CD3D12CommandSlot * GetComputeSlot();
+    CD3D12CommandSlot * GetCopySlot   (unsigned frameIndex);
+    CD3D12CommandSlot * GetComputeSlot(unsigned frameIndex);
 };