This adds an "alternate appearance system" using image overrides and a new
handler system.
Usage:
- Create a new `image()` casted as `var/image`
- Set image.override = 1
- Call mob.add_alt_appearance("appearance_ID", image, listOfMobsToDisplayTo).
- To remove an image, you use mob.remove_alt_appearance("appearance_ID")
Comes with one usage, cardborg costumes. They will now disguise you as a
standard borg to any silicon mobs and yourself.
As this is a per-client system, that means that only silicon mobs and you will
see yourself as a standard borg. All other mobs will see you as a silly
human in a cardborg costume.
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