Synchronize StyleLayer and Bucket/Buffer state

The state needed by the painter_xxx modules to render tiles is split up into two places:

• The map-wide Style / StyleLayer structure, which dictates the layers to be rendered, their types, and the layout/paint definitions that control their details.
• The per-source Tile objects (managed by SourceCache, populated by Sources, often via Worker requests), which hold Buckets (one per StyleLayer) of buffers containing vertex positions, colors, etc.

These can get out of sync when client code mutates the Style, e.g.:

• Using setPaintProperty to change a constant to a property function
• Removing and re-adding a layer https://github.com/mapbox/mapbox-gl-js/issues/3895#issuecomment-273565708
• (pending) changing the style's sprite sheet https://github.com/mapbox/mapbox-gl-js/pull/3662#issuecomment-262115507

In each of these cases, we have to work around the possibility that when the Style is updated, we could have one or more render cycles where some Tiles are stale, carrying Bucket state that corresponds to the pre-updated style layers.

Start by tracing how Style and StyleLayer updates reach Tile, SourceCache, Source, Worker, and the painter_xxx modules, then inspect Bucket state during render cycles. Reproduce the listed mutations, including setPaintProperty and removing and re-adding a layer. Done means stale Bucket state cannot be used after a style update.