Treat "images" as nested collection of a style, like sources or layers

From @jfirebaugh on November 20, 2014 2:17

We want something more convenient than the current automagical practice of appending permutations of .json, .png, and @2x to the sprite property and making requests to two separate resources which need to be in sync.

Options that have been proposed:

• External protobuf container A format inspired by glyph pbfs. The protobuf message would be a series of [metadata, image data] tuples. Metadata would be id, image format, pixel ratio, and sdf boolean. Open question: would we still have separate @1x and @2x files, or just use @2x images everywhere, or permit including multiple images per id (with different pixel ratios) in the same file?
• Inline JSON + base64 image data Include sprite data directly in the style. Metadata in JSON format, image data base64 encoded (can't really do better than that). Same question regarding @2x images as above.

@mapbox/gl

Copied from original issue: mapbox/mapbox-gl-style-spec#220

Start by reviewing the existing handling of the style's sprite property and the issue's two proposed container formats. The issue is done only when a concrete nested images collection design is agreed and the separate sprite resources no longer need to be kept in sync.