Separate large objects into their own tiles to protect frustum culling

[robhawkes]

Accurate frustum culling is being prevented on tiles that contain large objects that extend past the boundary of the tile (eg. rivers). This means that although the main tile may not be visible on the screen, the tile isn't unloaded from the scene until the larger object has left the screen.

By moving potentially large objects to another tile (rivers and water to start with), frustum culling should work as normal for the tile with smaller objects on (buildings, etc).