Support Staged Tile Loading

[josephbirkner]

To facilitate very responsive viewport updates, a staged loading model would make sense, once this feature is also supported by mapget. Generally, the approach would be that the mapget streams every requested tile two ore mroe times: Once in a very coarse state with few basic attributes, and then in a more refined state with all attributes. For some feature types, even a third iteration might make sense. Style rulesshould also be able to be applied to just a particular stage of a loaded tile.