a location for the data's ETag, or %NULL
a location to return the tile's last modification time, or %NULL
a #GAsyncResult provided to callback
a #GBytes containing the tile data, or %NULL if the tile was not in the cache or an error occurred
GException on failure.
Gets the tile data from a completed shumate_file_cache_get_tile_async() operation.
@modtime will be set to the time the tile was added to the cache, or the latest time it was confirmed to be up to date.
@etag will be set to the data's ETag, if present.