SymbolLayout lives on the worker thread and contains the persistent data needed for repeated placement. SymbolBucket contains the data generated during placement, and is transferred to the main thread for rendering.
This eliminates the risky sharing of GeometryTile::buckets between the main thread and worker thread during TileWorker::redoPlacement.
While here, rationalize the names of states a SymbolLayout may be in:
* Pending: Waiting for the necessary glyphs or icons to be available.
* Prepared: The potential positions of text and icons have been determined.
* Placed: The final positions have been determined, taking into account prior layers.
In TileWorker, all SymbolLayouts are stored in a single vector. Each SymbolLayout knows what state it is in, and TileWorker can easily determine how much progress it can make toward a final result.
* 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.
