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.
This pattern is already used in a ton of places such as https://github.com/OpenRA/OpenRA/blob/bleed/mods/ra/rules/structures.yaml#L437