Defrag: Collision with slopes underground

[chovelyoukai]

Describe the bug

It's possible for the player to collide with the sloped side of a brush under the ground, such as in hgb-sm10. It appears that part of the problem is that, due to gravity during slick, the player can get closer then 1/8th of a unit away from the ground. This doesn't seem to happen in q3df.

How To Reproduce

Do a rocket groundboost over a flat surface that has a sloped cut under it.

Issue Map

df_hgb-sm10

Expected Behavior

The player shouldn't hit the surface under the ground.

Operating System

N/A