Workaround stereo instancing issue with unity_StereoEyeIndex

Usually, in the stereo instancing path, we source unity_StereoEyeIndex from the fragment input structure's stereoTargetEyeIndex member. In fact, stereoTargetEyeIndex is needed as it is bound to the SV_RenderTargetArrayIndex semantic, which dictates which render target array slice is rasterized to.

However, for whatever reason, reading stereoTargetEyeIndex is not reliable in the fragment shader.  However, if I use the raw instance ID passed into the fragment shader, that works correctly.  More work probably needs to be done, but this is fine for now.

ScriptableRenderPipeline/LightweightPipeline/LWRP/Shaders/LightweightScreenSpaceShadows.shader

         half Fragment(Interpolators i) : SV_Target
         {
             UNITY_SETUP_INSTANCE_ID(i);
+#if !defined(UNITY_STEREO_INSTANCING_ENABLED)
+            // 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
+            // We can limit this workaround to stereo instancing for now.
+#endif
-            // 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
 #if defined(UNITY_STEREO_INSTANCING_ENABLED) || defined(UNITY_STEREO_MULTIVIEW_ENABLED)
             float deviceDepth = SAMPLE_TEXTURE2D_ARRAY(_CameraDepthTexture, sampler_CameraDepthTexture, i.texcoord.xy, unity_StereoEyeIndex).r;
 #else