on_tile_filled

When a cache fills a tile and the next source in the chain is a tile cache, it should call this function on the next source.

This way all the caches preceding a tile source in the chain get informed that the tile was used and can modify their metadata accordingly in the implementation of this function. In addition, the call of this function should be chained so within the implementation of this function it should be called on the next source in the chain when next source is a tile cache.

Parameters: