DisplayObject code no longer has to manage
UpdateContext::active_clip before calling out to children, because
each child still has access to its Gc pointer.
This was discovered almost by accident: @Dinnerbone noticed that `_global == null`, and surmised that `valueOf` was the culprit. However, this doesn't really make sense: `_global` is a bare object, so it shouldn't have a `valueOf` (and in practice, it doesn't).
The ultimate cause of such an odd comparison is as such:
1. Flash coerces the `_global` object to a numerical primitive by calling `valueOf`.
2. `_global.valueOf` is undefined. Flash handles calls to any uncallable value by literally just having it return `undefined`. In other words, all values are implicitly callable as empty functions.
3. `undefined` is then compared to `null`. These two values *are* equal under abstract equality (`==`). Hence, `_global == null`.
For comparison, modern ECMAScript engines throw errors on calls to uncallable values; and won't attempt to use an invalid `valueOf` to coerce objects. So none of this applies to, say, standard JavaScript in your browser.
In Flash, this also at least claims to halt ActionScript execution on the movie. No such implementation of AVM poisoning currently exists in Ruffle, primarily because it's unclear what gets poisoned and implementing some of these options isn't yet possible:
1. AVM (e.g. all movies) - we would need to make the AVM fail silently in this case. This is the most straightforward way to poison the movie, but I'm not sure if this is how Flash actually does it, or if it poisons...
2. Movie - the current structure of movies is incompatible with adding arbitrary data to them. We need to merge `moviefetch` in before we can attach data to loaded movies.
3. MovieClip - this would also be implementable but has problems. How do child MovieClips know that their parent has been poisoned, or vice versa? What if a movie clip is loaded from one movie and moved into another?
As a result, I have decided to hold off on implementing recursion poisoning until I know where it's supposed to go and how to implement that.
While I don't expect every host object to implement it correctly, this also gets rid of a lot of unnecessary `unwrap` calls that would allow a poorly-written Flash file to kill the interpreter.
Note that host objects that do so will *not* have access to their standard representation from within member functions - you will need to extend the interface to accomodate for them. This is due to long-standing limitations with type IDs and downcasting with types that bear lifetimes - it's entirely an unsafe operation and exposing such a facility to safe Rust is unsound. However, this will at least let us separate out several things from ScriptObject that don't need to be there for the time being.
`Object::function` now returns a pre-allocated function object. You may supply it an explicit prototype to have it linked into the function object (which is why we have to return a cell).
David Wendt