* Tidy handle damage code
* Fix environmental damage conditions
- Vehicle damage would never occur because it doesn't fire all the
hitpoints
- Missing sanity check on fall damage to check that unit isn't in a
vehicle
- Getting hit by a vehicle while on foot can be detected by checking for
a shooter
* Improve collision handling
- All collisions have a `_shooter` so we simply first check for that because burning doesn't and it could happen at any velocity
- We don't actually care what caused the collision because they should all cause the same type of wounding
- The exception is fall damage because we want to prioritise wounding the legs, we assume if the `_unit` had downward velocity and `_shooter == _unit` that it was fall damage (not necessarily always true, but almost always is and there doesn't look to be a more specific way to check).
Units accumulate damage in the local variables
"ACE_Medical_Engine_$HitXXXX", but these are never reset. The way these
vars are used is as a single-damage-event tracker, and the end result of
not resetting their value when they're done being used is that a
specific $HitXXXX can accumulate enough damage to cause the "select
wound with highest damage" code to always select _it_ for the next wound
event, even if it wasn't the primary target of the last wound event.
I.E. I get shot in the right leg badly. If I then get winged in the left
arm, that wound will end up on my right leg, because when the code runs,
it finds QGVAR($HitRightLeg) > QGVAR($HitLeftArm), and so it applies the
wound to the right leg.
