Support custom implementations of mobile units

[rossipaolo]

The same as #1598, this time for enemies.

ExampleMobileUnit.zip

[ajrb]

Generally seems fine, although this is not code I am familiar with so I may well miss something.

My understanding is that this allows the visuals for enemies to be changed into models, is that correct? Pretty sure the sprites can already be replaced.

[rossipaolo]

Yes, it allows to provide a custom implementation of enemy graphics. For example MasonFace is currently trying to use an opensource framework for npcs; i think he plans to do monsters next.

[rossipaolo]

[Interkarma]

I love where this is going. Thank you for help with review. I'll take a look at this one more closely when I can, and once conflicts resolved.

[Interkarma]

Hey @TheLacus! :) My apologies for losing track of this PR. I see you're still working on this one. Just ping me when out of draft and ready for review. I don't have any issues with how you're implementing this so far. I'll do my best to test and help find any issues once you're happy with it. Then we can move into master finally. :)

[rossipaolo]

Thank you! jefetienne also proposed to help testing, so hopefully we'll get this feature available to mod authors soon.

[rossipaolo]

I uploaded a simple mod that can help with testing. My understanding is that combat behaviour (DoMeleeDamage and ShootArrow) is strictly linked with billboard animation frames, so mod authors need to reimplement both asset handling and combat for specific enemies. I'm not sure if we can make it a bit easier to manage, but in any case i think is something that can be revised at a later time.

[Interkarma]

Thanks for all your hard work on this one @rossipaolo. I'm happy with all your changes and haven't been able to find any issues in base game. Really looking forward to watching this one evolve as modders start using this.