Compute the terrain altitude at the right place to determine whether the periapsis is underground

[eggrobin]

Possibly fix #3826. The preconditions on ComputeCollision are a bit of a mess, but so long as we are underground at last_time it should work. However, we were instead checking whether we would have been underground had we been on one branch of the φ=λ analemma. The height above terrain calculation used to actually compute the collision was correct, which is why it would show up in the right place (up to the issue of multiple intersections). This may explain some oddities as to whether we would try to compute the collision at all—though the underground periapsis thing is of course fundamentally deficient as a criterion.