Client side optimizations, autotile scheme update.

CHANGE
- Added immediate mode drawing to atlas image system.
- Neighbors are now computed for every tile, including "empty" tiles, so
that edge tiles could be included in autotiling schemes.
- Removed cell struct and replaced with a single inline function using
immediate tile drawing in order to fix an odd bug introduced by the
selective update scheme.  Will eventually switch back.
TODO
- Update widget system to use common metrics and map_metrics data
structures.
- Write editor widgets.
- Implement live autotiled map renderers (live for the editor and
precomputed for static terrain).