[DO NOT MERGE] Align vertex attributes at 4 byte boundaries

First part of https://github.com/mapbox/mapbox-gl-native/pull/590

Introduces a sprite atlas and moves icon rendering to use the sprite atlas.

Missing things:

- [ ] Move glyphs to the same system
- [ ] SDF icon support
- [ ] Investigate why rotating makes icons jitter

Is there a need for an utility class? It's pretty much unused currently.