Duke3D: Fix player Z interpolation on SE17.

• In SE17's first loop, the player's position is unconditionally backed up. This means jumping in the elevator, even while its not in motion is uninterpolated for the player.
• Removing these interpolation calls fixes that, however there is a hitch now when the elevator is going back down.
• If the sign of q is positive, we back up the player's Z coordinate to fix the aforementioned hitch.
• Also remove actor[j].bpos.z = sprite[j].z; call that's covered G_RecordOldSpritePos().