Fix small issue with GUI

dist/preview release/gui/babylon.gui.js

@@ -35,6 +35,7 @@ var BABYLON;
                     _this._resizeObserver = _this.getScene().getEngine().onResizeObservable.add(function () { return _this._onResize(); });
                     _this._onResize();
                 }
+                _this._texture.isReady = true;
                 return _this;
             }
             Object.defineProperty(AdvancedDynamicTexture.prototype, "background", {

gui/src/advancedDynamicTexture.ts

@@ -73,6 +73,8 @@ module BABYLON.GUI {
                 this._resizeObserver = this.getScene().getEngine().onResizeObservable.add(() => this._onResize());
                 this._onResize();
             }
+
+            this._texture.isReady = true;
         }
 
         public markAsDirty() {