Correct unit sizes during double-strike animations.
Incorrectly assumed active unit is always alive.

That isn't true during animations when the attacking unit is killed.
There was a longstanding bug where the AI would start to take its turn
before the current turn finished animating.
Attacking unit is not always alive when animating.

The animation happens after an attack is executed (so we can correctly
update the unit size labels).
Added first strike ability.

Also, updated the simple scenario to demonstrate this ability.