Added an inactive gameplay cvar for using the own shot origin or centers of other players' bboxes for force direction calculations

With this change opted into it's easy again to push players downwards or sideways with explosive shots as the force direction isn't calculated from target's camera on top of their model but from the center.

If opted in then self-damage's force is not calculated from eyes but from (current, not historical) shot origin height with no horizontal offset.

Total force and damage applied are unchanged in RadiusDamage because it's calculated before that from the nearest point on bbox.