Deduplicate images in sprite atlas

Nobody has claimed this yet.

@kkaefer and I chatted about the possibility of only adding unique sprite images to the atlas, and allowing multiple sprite names to reference the same location.

Spritezero can already generate spritesheets [with unique sprite images only](https://github.com/mapbox/spritezero/blob/master/index.js#L9), with multiple sprite names referencing the same location on the spritesheet. If a style uses this spritesheet layout option, can we use this data to do something similar in the atlas? Or would we analyze all the images to dedup regardless of whether the spritesheet is deduped?

/cc @mapbox/gl-core

Start by reading the linked spritezero index.js behavior and tracing how sprite images are added to the atlas in mapbox-gl-js. Determine whether to consume existing deduplication data or compare images directly; done means duplicate images occupy one atlas location while all sprite names still resolve correctly.

Written by the indexing model from the issue text.