_getInternalCacheRef(drawer) {
            const options = drawer.options;
            if (!options.usePrivateCache) {
                $.console.error("[CacheRecord.prepareInternalCacheSync] must not be called when usePrivateCache is false.");
                return undefined;
            }

            // we can get here only if we want to render incompatible type
            let internalCache = this[DRAWER_INTERNAL_CACHE];
            if (!internalCache) {
                internalCache = this[DRAWER_INTERNAL_CACHE] = {};
            }
            return internalCache[drawer.getId()];
        }
Get an internal cache reference for given drawer
@param {OpenSeadragon.DrawerBase} drawer
@return {OpenSeadragon.InternalCacheRecord|undefined}
@private