Modify _PrevViewProjMatrix for camera-relative rendering

ScriptableRenderPipeline/HDRenderPipeline/Camera/HDCamera.cs

             cameraPos = pos;
             screenSize = new Vector4(camera.pixelWidth, camera.pixelHeight, 1.0f / camera.pixelWidth, 1.0f / camera.pixelHeight);
 
+            if (ShaderConfig.s_CameraRelativeRendering != 0)
+            {
+                Matrix4x4 cameraDisplacement = Matrix4x4.Translate(cameraPos - prevCameraPos); // Non-camera-relative positions
+                prevViewProjMatrix *= cameraDisplacement; // Now prevViewProjMatrix correctly transforms this frame's camera-relative positionWS
+            }
+
             // Warning: near and far planes appear to be broken.
             GeometryUtility.CalculateFrustumPlanes(viewProjMatrix, frustumPlanes);
 

ScriptableRenderPipeline/HDRenderPipeline/HDRenderPipeline.cs

                 // If the flag hasn't been set yet on this camera, motion vectors will skip a frame.
                 hdcamera.camera.depthTextureMode |= DepthTextureMode.MotionVectors | DepthTextureMode.Depth;
 
-                // TODO: This is not safe ? as we don't use cmd buffer we can have a delay no ?
-                m_CameraMotionVectorsMaterial.SetVector(HDShaderIDs._CameraPosDiff, hdcamera.prevCameraPos - hdcamera.cameraPos);
-
                 // Setup stencil buffer
                 CoreUtils.DrawFullScreen(cmd, m_CameraMotionVectorsMaterial, m_VelocityBufferRT, m_CameraDepthStencilBufferRT, null, 0);
 

ScriptableRenderPipeline/HDRenderPipeline/HDStringConstants.cs

         public static readonly int _FilterKernelsBasic = Shader.PropertyToID("_FilterKernelsBasic");
         public static readonly int _HalfRcpWeightedVariances = Shader.PropertyToID("_HalfRcpWeightedVariances");
 
-        public static readonly int _CameraPosDiff = Shader.PropertyToID("_CameraPosDiff");
-
         public static readonly int _CameraDepthTexture = Shader.PropertyToID("_CameraDepthTexture");
         public static readonly int _CameraMotionVectorsTexture = Shader.PropertyToID("_CameraMotionVectorsTexture");
         public static readonly int _FullScreenDebugMode = Shader.PropertyToID("_FullScreenDebugMode");

ScriptableRenderPipeline/HDRenderPipeline/RenderPipelineResources/CameraMotionVectors.shader

         #include "../ShaderPass/VaryingMesh.hlsl"
         #include "../ShaderPass/VertMesh.hlsl"
 
-        float4 _CameraPosDiff;
-
         struct Attributes
         {
             uint vertexID : SV_VertexID;
             UpdatePositionInput(depth, UNITY_MATRIX_I_VP, UNITY_MATRIX_VP, posInput);
             float4 worldPos = float4(posInput.positionWS, 1.0);
             float4 prevPos = worldPos;
-
-        #if (SHADEROPTIONS_CAMERA_RELATIVE_RENDERING != 0)
-            prevPos -= _CameraPosDiff;
-        #endif
 
             float4 prevClipPos = mul(_PrevViewProjMatrix, prevPos);
             float4 curClipPos = mul(_NonJitteredViewProjMatrix, worldPos);