Synchronize StyleLayer and Bucket/Buffer state

[anandthakker]

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 Layers flash on map when one is removed and reapplied in 0.29.0 #3895 (comment)
• (pending) changing the style's sprite sheet Add style#setSprite method #3662 (comment)

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.

[jfirebaugh]

I'm marking #3949 and #3453 as duplicates of this issue, since this issue states the problem most generally, but note that those other issues have good discussion of specific aspects of this problem.

[anandthakker]

In #3453, @ansis suggests that we should aim for solution wherein all tiles being rendered have been laid out using the same view of the style.

If want to do that -- and even if we don't -- I think #4875 will make this much more tractable. Once we have an immutable Style object, having each Tile hold a reference to the Style with which it was created will be pretty straightforward.