Refactor Terrain - reduce coupling with simulation components and actual graphical rendering.

[wraitii]

This is a high-level task.

Terrain is very tightly coupled with the simulation in a variety of ways:

• TERRAIN_TILE_SIZE is used in many, many places. It has a value of 4 meters. The corresponding NAVCELL_PER_TILE constant is used to convert that back to meters, usually.
• Terrain height map and passability are very tightly coupled.
• LOS is coupled with terrain.
• Several components expect a height map (least of all the map generator and atlas).
• Terrain rendering is tile-based and detail cannot be added for now, and this is an increasing graphics issue.
• Terrain is strictly 2D.
• Textures are very coupled with our terrain specs (which makes some sense, but also limits our abilities).
• Overlays are coupled with terrain.
• Water rendering is coupled with terrain rendering
• We only support a single terrain implementation, simulation-wise and graphics-wise.

+ ... Possibly unidentified other ways ...

A variety of these issues can be fixed somewhat easily and will decrease coupling. Some will be harder, and some require some foresight (do we _need_ several terrain implementations?).

I intend to work on this on and off. Priority will be fixing simulation-terrain coupling, then seeing what can be done for the graphics and texture aspect of it, then seeing how that can be handled in Atlas and such.

[wraitii]

In 24980:

Decouple LOS resolution from terrain resolution

Introduce a LOS_TILE_SIZE, to replace usage of TERRITORY_TILE_SIZE in the LOS code.
This makes it possible to change the resolution of LOS/Terrain without affecting the other component.

Additional refactoring:

• LosTile has been renamed LosRegion (it's more comparable to the hierarchical pathfinder regions/spatial subdivisions)
• LosState explicitly refers to "los vertices" instead of terrain vertices.

Refs #5566

Differential Revision: ​https://code.wildfiregames.com/D3076