Prototype "bucket merging" for Circle layer

@ChrisLoer is already working on this.

Since Oct 19, 2018.

Layers that aren't clipped at tile boundaries (e.g. Circle and Symbol) can't guarantee consistent draw-order sorting of features that cross the tile boundary. See https://github.com/mapbox/mapbox-gl-js/issues/7111.

One of our proposals for addressing this problem is to try to merge all of the tiles for a layer into a single "mega tile" so that all of the features can be drawn in a single draw call. This would give us the technical capability to control the draw order across all tiles, although we'd still need a separate mechanism to actually determine which order to use (e.g. sorting based on y-position would work fine, but the current approach of "sort based on order in the data" wouldn't work outside of a single-tile context).

As a first step, we plan to prototype "bucket merging" for circle layers: our goal is to figure out the performance characteristics of this approach. The approach will require re-uploading the entire "global" buffer every time any tile changes, and it will also require converting tile coordinates into some kind of global coordinate system -- we need to see if we can keep those costs low enough to make this functionality performant.

cc @ansis @tmpsantos @chloekraw