function getBackingStore (id) {
  for (var i=0, len=_backingStores.length; i < len; i++) {
    if (_backingStores[i].id === id) {
      return _backingStores[i].canvas;
    }
  }
  return null;
}
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}