code: function getBackingStore (id) {
    for (var i=0, len=_backingStores.length; i < len; i++) {
      if (_backingStores[i].id === id) {
        return _backingStores[i].canvas;
      }
    }
    return null;
  }
docs: Maintain a cache of backing <canvas> for RenderLayer's which are accessible
  through the RenderLayer's `backingStoreId` property.
  
  @param {String} id The unique `backingStoreId` for a RenderLayer
  @return {HTMLCanvasElement}