Iterating through tiles in viewports now works fine. You can pass in a custom device as the data source for the tile map.
The iterator is not included anywhere yet so it's completely untested. Next step is to define the relationship between views and layers, so iterators can be put into the class that will be responsible to spawn them.
Deferring the data loading task to lower level classes. test_tiler removed as Tyler won't require the user to derive from it anymore.
