Added missing material typing on Object3D

src/core/Object3D.d.ts

@@ -39,6 +39,11 @@ export class Object3D extends EventDispatcher {
   name: string;
 
   type: string;
+  
+  /**
+   * The material of the object.
+   */
+  material: Material;
 
   /**
    * Object's parent in the scene graph.