Moved Pow4 to srp library.

ScriptableRenderPipeline/Core/CoreRP/ShaderLibrary/Common.hlsl

     return x * x * (3.0 - (2.0 * x));
 }
 
+real Pow4(real x)
+{
+    return (x * x) * (x * x);
+}
+
 // ----------------------------------------------------------------------------
 // Texture utilities
 // ----------------------------------------------------------------------------