Shader material serialization

Temp solution until there will be a serialize method on the shader
material.
I also wonder why the materials were serialized twice...

src/Tools/babylon.sceneSerializer.ts

@@ -259,6 +259,8 @@
             var material: Material;
             for (index = 0; index < scene.materials.length; index++) {
                 material = scene.materials[index];
+				//ShaderMaterial is not yet being serialized.
+				if(material instanceof ShaderMaterial) continue;
                 serializationObject.materials.push(material.serialize());
             }
 
@@ -269,11 +271,6 @@
                 serializationObject.multiMaterials.push(multiMaterial.serialize());
             }
 
-            for (index = 0; index < scene.materials.length; index++) {
-                material = scene.materials[index];
-                serializationObject.materials.push(material.serialize());
-            }
-
             // Skeletons
             serializationObject.skeletons = [];
             for (index = 0; index < scene.skeletons.length; index++) {