surface: Make Tank follow terrain on RoundAstrobj
cubex: Optimize skirt length for hiding gaps to reduce pixel fills

The skirt length is calculated by terrain's variance so that it's enough
to hide gaps between LODs but not too long.
surface: Derive gradient of RoundAstrobj's terrain for Tank's orientation

This naive way to obtain gradient is inefficient, but we settle for that
because sdnoise's not working properly with fractal noise.