Integrate sprite jsons into stylesheet

[peterqliu]

Is there a compelling reason we keep sprite.json and sprite@2x.json as separate files from the stylesheet, rather than as JSON objects within?

It adds more (unnecessary?) moving parts, and introduces the potential for missing links

[incanus]

Broader discussion happening over in mapbox/mapbox-gl-style-spec#220.

The reason we have sprites is similar to the web (less resources to handle) but also optimizing texture loads in GL since you can bind one image, then use the offsets to map textures.

Since we also will eventually support runtime addition and subtraction of sprites, I'd say JSON's days are numbered.

[jfirebaugh]

Yeah let's keep the discussion over on the style-spec issue.