Appearance Weakrefs

[amylizzle]

Fairly substantial rewrite of appearance handling.

There are now two appearance classes: MutableIconAppearance and ImmutableIconAppearance

MutableIconAppearance is the class you use for mutable appearance stuff. They are not sent to the client. To send them to the client, you must either use AtomManager.SetAppearance() or if you have a really really good reason use ServerAppearanceSystem.AddAppearance().

ImmutableIconAppearance is the class that is sent to the client. It cannot be edited. If you want to edit it, you need to create a mutable copy (which is a MutableIconAppearance). These are held only as weakrefs in the ServerAppearanceSystem, and are removed from connected clients when they are garbage collected. This means that if you want to keep it alive you must have a hard ref to it somewhere. Any appearance (mutable or no) which has an overlay/underlay counts as a hard ref for that overlay/underlay.

The client has copies of the ImmutableIconAppearance and can use ToMutable() to edit them locally for animations and such.

Appearance ID is now just the ImmutableIconAppearance's hash as generated on the server and stored in the ImmutableIconAppearance.

KNOWN BUG: /turf animations don't apply, it just sets the final state of the turf appearance. Turf animations can go in another PR.