Internal function calls such as `toString` and event handlers
always used the base clip from the function itself, so add an
`ExecutionReason::InternalCall` to signify this and pass it along
to `call_method`.
Remove the `swf_version` parameter from `Activation` constructors,
because this was incorrectly using the global or root SWF version
most times.
Instead, grab the SWF version for the activation directly from the
base clip.
Use prototype depths instead. Most calls passed `base_proto = None`,
which is equivalent to `depth = 0`, and is now the default.
The few other cases were adapted to use `Executable::exec` directly,
where `depth` can be specified manually.
When reading an SWF, search for FileAttributes and
SetBackgroundColor and return this along with the header data
because it's useful (in particular, the AS3 flag).
Notably, all of the `Avm1` "run stack frame" functions can no longer take a self parameter as the update context they will be getting also has that same parameter. Ergo, they're associated functions that get the moral equivalent of self from the update context.
This also introduces a new `Activation::from_stub` which creates a stub frame that runs everything on the main movie in layer 0. This significantly reduces boilerplate code elsewhere in the project.
The process of constructing an `Activation` now involves calling `UpdateContext.reborrow`, which "sheds" a lifetime by copying all of the borrows into a new "owned" context with that lifetime.
Likewise, to call out to functions that don't need an `Activation`, just borrow the context out of the current activation. You can also construct child-frame activations by reborrowing the parent activation's context.