[Added] GetModifiedDamage + ModifyDamage Expansion

[Major Cooke]

Pull Request

DamageMobj now passes in the inflictor, source, and its flags (for RadiusDamageFactor checking via DMG_EXPLOSION) to powerups - and can also be passed in via the modder's side too.

[Major Cooke]

Another small PR.

I overlooked the fact that PowerDamage didn't have any way to get the actual target being damaged because it passed in the original source, which is redundant. So I repurposed it to pass in the target in the internal DamageMobj function instead. This way the powerup can see whose being damaged.

However, if you think I should just pass in the target directly as an additional parameter, I can change it to do that. That would make things more clear, while the above is simplest. Which would be preferred? I'd hate to make it more confusing for others.

[Rachael]

Both PR's here got merged, marking accordingly.