Fix nifbullet shape transformation
Make an actor fly when it has a levitate effect
Ensure updated skeleton bone placement matches in world space

Objects attached to actors (shirts, robes, etc) do not require the same node
hierarchy as the character root. So to ensure proper placement, we need to set
the bone target's derived transformation using the source bone's derived
transformation (which in turn means we need to work up from the root, to ensure
the bone's parents are properly placed).
Simplify newtrace a bit
Set the character as being on the ground when colliding with a shallow enough slope
Fix stepping and vertical force accumulation