Fix up camera-relative rendering wrt stereo

Find center position between left and right eye, and use that to generate camera-relative offsets for the other view matrices.

ScriptableRenderPipeline/HDRenderPipeline/HDRP/Camera/HDCamera.cs

 
         public Matrix4x4[] viewMatrixStereo;
         public Matrix4x4[] projMatrixStereo;
+        public Vector4 centerEyeTranslationOffset;
 
         // Non oblique projection matrix (RHS)
         public Matrix4x4 nonObliqueProjMatrix
             var gpuView = camera.worldToCameraMatrix;
             var gpuNonJitteredProj = GL.GetGPUProjectionMatrix(nonJitteredCameraProj, true);
 
+            // In stereo, this corresponds to the center eye position
-
-            // TODO: Delete this?
-            var relPos = pos; // World-origin-relative
-                relPos = Vector3.zero; // Camera-relative
             }
 
             var gpuVP = gpuNonJitteredProj * gpuView;
 
             if (ShaderConfig.s_CameraRelativeRendering != 0)
             {
-                // Zero out the translation component.
-                stereoCombinedViewMatrix.SetColumn(3, new Vector4(0, 0, 0, 1));
+                // This is pulled back from the center eye, so set the offset
+                var translation = stereoCombinedViewMatrix.GetColumn(3);
+                translation += centerEyeTranslationOffset;
+                stereoCombinedViewMatrix.SetColumn(3, translation);
             }
 
             viewMatrix = stereoCombinedViewMatrix;
             for (uint eyeIndex = 0; eyeIndex < 2; eyeIndex++)
             {
                 viewMatrixStereo[eyeIndex] = camera.GetStereoViewMatrix((Camera.StereoscopicEye)eyeIndex);
-                if (ShaderConfig.s_CameraRelativeRendering != 0)
+
+                projMatrixStereo[eyeIndex] = camera.GetStereoProjectionMatrix((Camera.StereoscopicEye)eyeIndex);
+                projMatrixStereo[eyeIndex] = GL.GetGPUProjectionMatrix(projMatrixStereo[eyeIndex], true);
+            }
+
+            if (ShaderConfig.s_CameraRelativeRendering != 0)
+            {
+                var leftTranslation = viewMatrixStereo[0].GetColumn(3);
+                var rightTranslation = viewMatrixStereo[1].GetColumn(3);
+                var centerTranslation = (leftTranslation + rightTranslation) / 2;
+                var centerOffset = -centerTranslation;
+                centerOffset.w = 0;
+
+                // TODO: Grabbing the CenterEye transform would be preferable, but XRNode.CenterEye
+                // doesn't always seem to be valid.
+
+                for (uint eyeIndex = 0; eyeIndex < 2; eyeIndex++)
-                    // Zero out the translation component.
-                    viewMatrixStereo[eyeIndex].SetColumn(3, new Vector4(0, 0, 0, 1));
+                    var translation = viewMatrixStereo[eyeIndex].GetColumn(3);
+                    translation += centerOffset;
+                    viewMatrixStereo[eyeIndex].SetColumn(3, translation);
-
-                projMatrixStereo[eyeIndex] = camera.GetStereoProjectionMatrix((Camera.StereoscopicEye)eyeIndex);
-                projMatrixStereo[eyeIndex] = GL.GetGPUProjectionMatrix(projMatrixStereo[eyeIndex], true);
+                centerEyeTranslationOffset = centerOffset;
             }
 
             // TODO: Fetch the single cull matrix stuff

ScriptableRenderPipeline/HDRenderPipeline/HDRP/Lighting/LightLoop/LightLoop.cs

                         //TODO: Do not call ToArray here to avoid GC, refactor API
                         int[]   shadowRequests = m_ShadowRequests.ToArray();
                         int[]   shadowDataIndices;
-                        
+
-                        // For stereo, this is fine as long as this matches the head position, along with
-                        // our unified V/P/VP matrices working from that head position.
+                        // which corresponds to the 'center eye' position. There might be some dissonance
+                        // with the combined view pullback position.
+                        // It is used in ShadowAtlas.Reserve, for camera relative rendering, 
+                        // and ShadowManager.PruneShadowCasters.  The uses seem appropriate, but worth keeping an eye on.
                         m_ShadowMgr.ProcessShadowRequests(m_FrameId, cullResults, camera, ShaderConfig.s_CameraRelativeRendering != 0, cullResults.visibleLights,
                             ref shadowRequestCount, shadowRequests, out shadowDataIndices);