HDRenderLoop: Fix Texture settings, change it to struct again

Assets/ScriptableRenderLoop/HDRenderLoop/Editor/HDRenderLoopInspector.cs

 
             if (EditorGUI.EndChangeCheck())
             {
+                renderLoop.textureSettings = textureParameters;
                 EditorUtility.SetDirty(renderLoop); // Repaint
             }
             EditorGUI.indentLevel--;

Assets/ScriptableRenderLoop/common/TextureSettings.cs

 namespace UnityEngine.Experimental.ScriptableRenderLoop
 {
     [System.Serializable]
-    public class TextureSettings
+    public struct TextureSettings
     {
         public int spotCookieSize;
         public int pointCookieSize;