integrate new terrain into the current game loop

minimum integration of new terrain. could just import new terrain mesh into the game loop.
Terrain integration will be important for future development, and should be somewhat in place after sprint 1.

requirments:
- new terrain mesh added to the assets
- current system is integrated with the new terrain mesh
- game flow is tested with current system
- navmesh is built
- navmesh agents and other entities work as previous with new terrain mesh