Cleanup for weapon explosions

[StephenCWills]

With the changes in this PR, weapon explosions only do the collision check once, in AddWeaponExplosion(). I set the dontDeleteOnCollision flag to prevent the missile from immediately deleting itself. This means the missile animation will always play to completion regardless of whether the elemental damage strikes the target.

I modified DoAttack() to use the didhit flag to determine whether the missile should attempt to do damage to the target. Just like with elemental arrows, if the physical attack doesn't hit, the elemental attack doesn't hit either. In order to skip the collision check but still play the missile animation, DoAttack() simply sets the missile damage to zero if the physical attack missed. AddWeaponExplosion() will skip the collision check entirely if it sees that the missile damage is zero.

This resolves #7571

[obligaron]

didhit is calculated by PlrHitMonst / PlrHitPlr and this calculates if the hit can fail. This check is used to calculate the damage (0 if the hit failed).
In AddWeaponExplosion we call CheckMissileCol and this calls Plr2PlrMHit / MonsterMHit. These functions also calculate if the missile hits or not.
Does the weapon explosion now have to pass two hit checks instead of one? 🤔

[StephenCWills]

Yes, I did this on purpose to match the behavior of elemental arrows. For arrows, the elemental explosion can't even spawn unless the arrow connects with a target so we can't really fix that behavior without refactoring the collision logic or adding yet another parameter.

[obligaron]

I looked at the arrow logic and I'm with you.
It just showed me that we are changing / nerfing the elemental damage. But I'm fine with aligning it with the way elemental arrows are handled.