Sharing StyleLayers between WorkerTiles is dangerous

Nobody has claimed this yet.

`StyleLayer`s are shared between `WorkerTile`s, via the worker `StyleLayerIndex`. But `StyleLayer`s contain state consisting of the evaluated result of style properties at a certain zoom level, and `WorkerTile`s may have different zoom levels.

Currently, we avoid any issues that this shared state could cause by carefully calling `StyleLayer#recalculate` at the entry point to every async callback in `WorkerTile`. But this is relatively fragile and error prone, and also requires reevaluation of previously-evaluated properties.

A better solution would be to have each `WorkerTile` call `StyleLayer#recalculate` once for each necessary layer, and store the logical result state independently of any other `WorkerTile`s. This would likely require a significant refactor of the internals of style recalculation (refs #2739, #3044).

Start by reading the WorkerTile, StyleLayer, and worker StyleLayerIndex implementations to trace how shared layers and asynchronous callbacks use recalculated state. Compare the related discussions in issues #2739 and #3044, then define completion as each WorkerTile maintaining independent recalculation state without fragile callback-level reevaluation.

Written by the indexing model from the issue text.