Implement terrain heightmaps

![grid](https://cloud.githubusercontent.com/assets/167819/4948922/4490f21a-66ae-11e4-8140-94d6308e9032.png)

This adds engine and map format support for terrain heightmaps using the metadata introduced in #6855.
A new TS test map is included to demonstrate these capabilities.

The focus of this is specifically on the terrain: there is a bunch of gameplay code that is not yet height aware, and this leads to a bunch of bugs on the test map (such as the glitchy shroud, aircraft that fly underground, building placement, etc).  These can be fixed in future PRs.