get defaultOptions() {
            return {
                // use detached cache: our type conversion will not collide (and does not have to preserve CPU data ref)
                usePrivateCache: true,
                preloadCache: false,
            };
        }
The parent element of this Drawer instance, passed in when the Drawer was created.
The parent of {@link OpenSeadragon.WebGLDrawer#canvas}.
@member {Element} container
@memberof OpenSeadragon.WebGLDrawer#