feat(VR): DLSSperf — render engine at upscaled resolution

features/DLSSperf/Shaders/DLSSperf/BilinearCopyPS.hlsl

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+// BilinearCopyPS.hlsl — DLSSperf downscale pass
+// Box 3×3 filter: testTexture (3k) → kMAIN (1k)
+// For 3:1 downscale, each output pixel averages the 3×3 source region,
+// ensuring all DLSS output pixels contribute (vs bilinear's 2×2 coverage).
+// Reuses UpscaleVS.hlsl for fullscreen triangle generation (SV_VertexID)
+
+#include "Upscaling/UpscaleVS.hlsl"
+
+#if defined(PSHADER)
+
+typedef VS_OUTPUT PS_INPUT;
+
+SamplerState LinearSampler : register(s0);
+Texture2D<float4> SourceTex : register(t0);
+
+float4 main(PS_INPUT input) : SV_Target
+{
+	float2 srcSize;
+	SourceTex.GetDimensions(srcSize.x, srcSize.y);
+	float2 texelSize = 1.0 / srcSize;
+
+	float4 sum = 0;
+	[unroll] for (int y = -1; y <= 1; y++)
+		[unroll] for (int x = -1; x <= 1; x++)
+			sum += SourceTex.SampleLevel(LinearSampler, input.TexCoord + float2(x, y) * texelSize, 0);
+	return sum * (1.0 / 9.0);
+}
+
+#endif