Add SH shading to gaussian splat renderer

engine/assets/builtin/shaders/gaussian-splat.shader

@@ -11,8 +11,10 @@ Shader "Builtin Gaussian Splat"
         float4x4 gProjectionMatrix;
         float4x4 gViewMatrix;
         float4x4 gModelMatrix;
+        float4x4 gWorldToObjectMatrix;
         float4 gCameraRight;
         float4 gCameraUp;
+        float4 gCameraWorldPos;
         float4 gScreenParams;
         float4 gSplatParams;
     };