[not ready] add center and extent to vector_layers

[GretaCB]

This is to set up for zoom-to in Mapbox Studio Style UI for remote sources.

/cc @yhahn

[coveralls]

Changes Unknown when pulling 39db805 on more_metadata into * on master*.

[GretaCB]

coveralls icons is creepy

[yhahn]

Let's hold on merging here -- a few thoughts atm:

• doc task here Document vector_layers extension tilejson-spec#14
• extent? or bounds?
• we'll want to add validation for new vector_layers keys to https://github.com/mapbox/tilelive.js/blob/master/lib/tilelive.js#L250-L260

[tmcw]

[GretaCB]

@miccolis ^^^ mapbox uniforms

@yhahn, cool, I'll hold off and jump on some other things in the meantime.

[rclark]

This is pretty stale. Any new thinking here or can we close?

[GretaCB]

This would still be a nice feature to include in mapbox-studio. Though I am not sure what the trajectory is/priorities are on Studio currently

cc @yhahn @samanpwbb

[yhahn]

@rclark @GretaCB my moral compass spins on this because it seems like in many ways what we're trying to capture is a tilejson description for every layer in vector_layers -- e.g. answering the question:

If this layer were a standalone source, what would its center/minzoom/maxzoom/bounds be?

I think it's an interesting way of thinking about layers and datasources this way and I've just not come to a settled place on what this means about how we think about vector sources.