[DNM/TM] External organ icon handling tweak

[Vi3trice]

What Does This PR Do

Adds an early return for update_icon, make external organs call a separate update proc instead of update_icon, and also make them not emissive blockers. Not realistic, but the odds of having a limb lying around also blocking an item are low and this cuts down on them managing the blocker. This doesn't affect the mobs and they still block emissives.

Why It's Good For The Game

External organs don't need to go through the whole update_icon process as its handled by the mob itself. Likewise if something doesn't want to update its icon on purpose, it shouldn't be sending signals that it just did.

Testing

Shot at unathis, damage display is still handled properly. Also stuck a stack trace in update_icon for debugging purposes.

Changelog

N/A