function destroyBillboards(billboards) {
  const length = billboards.length;
  for (let i = 0; i < length; ++i) {
    if (billboards[i]) {
      billboards[i]._destroy();
    }
  }
}
Cache of loaded billboard images.
@private
@memberof BillboardCollection.prototype
@type {Map<string, BillboardTexture>}
@readonly