The Y coordinate at which to fire a projectile is currently assumed to
be the target's current Y, which is incorrect if the target is moving on
a slope.
This fixes that.
Note that this was purely visual, since projectiles still hit the target
regardless, as the height component is totally ignored, even if the
projectile is underground (in fact, the projectile's position is not
known in DelayedDamage::MissileHit, which just assumes it lands where it
said it would when fired).
As noted by bb in f737831167Fixes#5939
Differential Revision: https://code.wildfiregames.com/D3425
This was SVN commit r24766.