UBackgroundBlur crashes at PSO creation under D3D12 SM6 when stereo rendering is enabled

Any Slate widget that triggers the Slate post-process blur (e.g., a UBackgroundBlur with non-zero strength) hits a fatal D3D12 CreateGraphicsPipelineState failure on first render. The fatal is funneled through PipelineStateCache.cpp:707 ("Shader compilation failures are Fatal"), but the underlying error is E_INVALIDARG (0x80070057) from a VS/PS signature linkage mismatch, not an HLSL compile error.

1. Run the attached project in the editor
2. Create a UMG widget blueprint
3. Add a BackgroundBlur component
4. Set the blur strength to a non-zero value.