Use DrawProcedural instead of Blit for Screen-space Shadow Map

Because of stereo instancing issues with null source Blit, we can try using DrawProcedural.

Everything seems to work ok, except for some bizarre unity_StereoEyeIndex functionality inside of the fragment shader during instancing.

ScriptableRenderPipeline/LightweightPipeline/LWRP/LightweightPipeline.cs

             // doesn't like null sources when trying to determine a stereo-ized blit.  So for proper
             // stereo functionality, we use the screen-space shadow map as the source (until we have
             // a better solution).
-            cmd.Blit(m_ScreenSpaceShadowMapRT, m_ScreenSpaceShadowMapRT, m_ScreenSpaceShadowsMaterial);
+            //cmd.Blit(m_ScreenSpaceShadowMapRT, m_ScreenSpaceShadowMapRT, m_ScreenSpaceShadowsMaterial);
+            //cmd.Blit(null, m_ScreenSpaceShadowMapRT, m_ScreenSpaceShadowsMaterial);
+
+            // Can't use CoreUtils.DrawFullScreen yet because it doesn't support stereo texture arrays yet
+            SetRenderTarget(cmd, m_ScreenSpaceShadowMapRT);
+            cmd.DrawProcedural(Matrix4x4.identity, m_ScreenSpaceShadowsMaterial, 0, MeshTopology.Triangles, 3, 1);
 
             if (LightweightUtils.HasFlag(frameRenderingConfiguration, FrameRenderingConfiguration.Stereo))
                 context.StartMultiEye(m_CurrCamera);

ScriptableRenderPipeline/LightweightPipeline/LWRP/Shaders/LightweightScreenSpaceShadows.shader

             UNITY_VERTEX_INPUT_INSTANCE_ID
         };
 
+        struct FullScreenInput
+        {
+            uint vertexID : SV_VertexID;
+            UNITY_VERTEX_INPUT_INSTANCE_ID
+        };
+
         struct Interpolators
         {
             half4  pos      : SV_POSITION;
         };
 
-        Interpolators Vertex(VertexInput i)
+        //Interpolators Vertex(VertexInput i)
+        Interpolators Vertex(FullScreenInput i)
         {
             Interpolators o;
             UNITY_SETUP_INSTANCE_ID(i);
-            o.pos = TransformObjectToHClip(i.vertex.xyz);
+            //o.pos = TransformObjectToHClip(i.vertex.xyz);
+            o.pos = GetFullScreenTriangleVertexPosition(i.vertexID);
-            o.texcoord.xy = UnityStereoTransformScreenSpaceTex(i.texcoord.xy);
+            //o.texcoord.xy = UnityStereoTransformScreenSpaceTex(i.texcoord.xy);
+            o.texcoord.xy = GetFullScreenTriangleTexCoord(i.vertexID);
             o.texcoord.zw = projPos.xy;
 
             return o;
             UNITY_SETUP_INSTANCE_ID(i);
             UNITY_SETUP_STEREO_EYE_INDEX_POST_VERTEX(i);
 
-            // TODO
-            // declare texture correctly as tex2darray
-            // pass in stereo eye index in correctly so it can sample texture
-            // Fix up sampling from a depth texture array
+            float2 adjTexCoords = UnityStereoTransformScreenSpaceTex(i.texcoord.xy);
+
-            float deviceDepth = SAMPLE_TEXTURE2D_ARRAY(_CameraDepthTexture, sampler_CameraDepthTexture, i.texcoord.xy, unity_StereoEyeIndex).r;
+            // Completely unclear why i.stereoTargetEyeIndex doesn't work here, considering
+            // this has to be correct in order for the texture array slices to be rasterized to
+            unity_StereoEyeIndex = i.instanceID;
+            float deviceDepth = SAMPLE_TEXTURE2D_ARRAY(_CameraDepthTexture, sampler_CameraDepthTexture, adjTexCoords, unity_StereoEyeIndex).r;
-            float deviceDepth = SAMPLE_DEPTH_TEXTURE(_CameraDepthTexture, sampler_CameraDepthTexture, i.texcoord.xy);
+            float deviceDepth = SAMPLE_DEPTH_TEXTURE(_CameraDepthTexture, sampler_CameraDepthTexture, adjTexCoords);
 #endif
 
 #if UNITY_REVERSED_Z