* Renamed {Source,Tile}Observer::onNeedsRepaint to onTileUpdated. Messages should be in terms of what happened to the observed object, not in terms of what the observer needs to do. This also removes a confusing overlap of virtual methods on StyleObserver.
* Added style::Observer::onUpdate(Update). This is also a violation of the above rule, but I'm hopeful that it will disappear when update batching is implemented.
Tile is now the main base class; RasterTile, VectorTile, etc are its subclasses. GeometryTileData and its subclasses form the piece that's passed to the worker.
