Gets the data for the tile at the given coordinates.
Some data sources may return data multiple times. For example, [class@TileDownloader] may return data from a cache, then return updated
data from the network. [signal@ShumateDataSource:ShumateDataSource:received-data] is emitted each time data is received, then
callback is called after the last update.
| this |
a [class@DataSource] |
| x |
the X coordinate to fetch |
| y |
the Y coordinate to fetch |
| zoom_level |
the Z coordinate to fetch |
| cancellable |
a Cancellable |
| callback |
a TaskReadyCallback to execute upon completion |
| user_data |
closure data for |