[Stage] wmodeGPU (false) and showDefaultContextMenu (stub) properties
[Vector] Subtle refcounting error

Array.at() returns an _R<ASObject> which got destructed (setting the
refcount of the returned object to 0) before incRef() on the next
line.
[test] Exclude testcases that need imports from shell_toplevel

shell_toplevel uses API versioning which is not yet supported. See
doc/apiversioning.txt in Tamarin sources.