Add missing instance variable declaration, thanks @elemoine

src/ol/renderer/webgl/webglmaprenderer.js

@@ -117,6 +117,12 @@ ol.renderer.webgl.Map = function(container, map) {
   this.canvas_.className = 'ol-unselectable';
   goog.dom.insertChildAt(container, this.canvas_, 0);
 
+  /**
+   * @private
+   * @type {boolean}
+   */
+  this.renderedVisible_ = true;
+
   /**
    * @private
    * @type {ol.Size}