Target height is not computed correctly for targets moving on a slope

[wraitii]

Per the title.

Units currently aim at the foot of their target, and this can miss if the target is moving up, since the projectile will land in the ground.

NB: this is visual only, projectiles are "teleported" where they needed to be anyways.

[wraitii]

In 24766:

Fix target height computation when launching projectiles.

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 rP24701

Fixes #5939

Differential Revision: ​https://code.wildfiregames.com/D3425