getViewer(drawer=null, props={}) {
        drawer = drawer || this.getDrawer();
        return OpenSeadragon.extend(new class extends OpenSeadragon.EventSource {
            forceRedraw () {}
            drawer = drawer
            tileCache = new OpenSeadragon.TileCache()
        }, props);
    }
Get mocked viewer: it has not all props that might be required. If your
tests fails because they do not find some props on a viewer, add them here.

Requires a drawer reference (mock or real). Automatically created if not provided.
@return {OpenSeadragon.Viewer}