Add Terrain Heightmap Loading

[Indy2222]

This is a continuation of #7.

• Modify map TAR file so it contains an entry with the heightmap (NPZ entry?).
• Split the terrain to a grid of meshes:
  • This allows better camera frustum culling (only relevant parts will be rendered).
  • Also, various intersection algorithms will be faster due to AABB pre-filtering.
• Modify line rendering (originally implemented in Display arrows from factory spawn to target #489) so they are rendered correctly on uneven terrain.

This requires modification of object placement and movement in the map so everything is positioned correctly on the surface.

Benchmark the solution: if slow, RTIN or other mesh simplification techniques might be required.