Adding references in playground for cell material

Playground/debug.html

@@ -34,6 +34,7 @@
     <script src="https://preview.babylonjs.com/materialsLibrary/babylon.gridMaterial.min.js"></script>
     <script src="https://preview.babylonjs.com/materialsLibrary/babylon.shadowOnlyMaterial.min.js"></script>
     <script src="https://preview.babylonjs.com/materialsLibrary/babylon.customMaterial.min.js"></script>
+    <script src="https://preview.babylonjs.com/materialsLibrary/babylon.cellMaterial.min.js"></script>
 
     <script src="https://preview.babylonjs.com/proceduralTexturesLibrary/babylon.brickProceduralTexture.min.js"></script>
     <script src="https://preview.babylonjs.com/proceduralTexturesLibrary/babylon.cloudProceduralTexture.min.js"></script>

Playground/frame.html

@@ -23,6 +23,7 @@
     <script src="https://preview.babylonjs.com/materialsLibrary/babylon.gridMaterial.min.js"></script>
     <script src="https://preview.babylonjs.com/materialsLibrary/babylon.shadowOnlyMaterial.min.js"></script>
     <script src="https://preview.babylonjs.com/materialsLibrary/babylon.customMaterial.min.js"></script>
+    <script src="https://preview.babylonjs.com/materialsLibrary/babylon.cellMaterial.min.js"></script>
 
     <script src="https://preview.babylonjs.com/proceduralTexturesLibrary/babylon.brickProceduralTexture.min.js"></script>
     <script src="https://preview.babylonjs.com/proceduralTexturesLibrary/babylon.cloudProceduralTexture.min.js"></script>

Playground/index.html

@@ -34,6 +34,7 @@
     <script src="https://preview.babylonjs.com/materialsLibrary/babylon.gridMaterial.min.js"></script>
     <script src="https://preview.babylonjs.com/materialsLibrary/babylon.shadowOnlyMaterial.min.js"></script>
     <script src="https://preview.babylonjs.com/materialsLibrary/babylon.customMaterial.min.js"></script>
+    <script src="https://preview.babylonjs.com/materialsLibrary/babylon.cellMaterial.min.js"></script>
 
     <script src="https://preview.babylonjs.com/proceduralTexturesLibrary/babylon.brickProceduralTexture.min.js"></script>
     <script src="https://preview.babylonjs.com/proceduralTexturesLibrary/babylon.cloudProceduralTexture.min.js"></script>