get
Returns the Tile for the specified coordinates.
Called on a background thread; several calls may run at the same time, so implementations must be thread-safe.
Return
The tile, NO_TILE if there is no tile at these coordinates (it is not requested again), or null if it cannot be loaded right now (it is retried later)
Parameters
x
Tile column (0 to 2^zoom - 1)
y
Tile row (0 to 2^zoom - 1)
zoom
Zoom level (0-19)