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:

- [x] Move glyphs to the same system
- [x] SDF icon support
- [x] Investigate why rotating makes icons jitter
- [x] Fix Retina
`uninitialized` is not used anywhere