set originalCacheKey(value) {
        throw "Original Cache Key cannot be managed manually!";
    }
By default equal to tile.cacheKey, marks a cache associated with this tile
that holds the cache original data (it was loaded with). In case you
change the tile data, the tile original data should be left with the cache
'originalCacheKey' and the new, modified data should be stored in cache 'cacheKey'.
This key is used in cache resolution: in case new tile data is requested, if
this cache key exists in the cache it is loaded.
@member {String} originalCacheKey
@memberof OpenSeadragon.Tile#