Marco Trevisan authored May 27, 2019 and Marco Trevisan committed Jun 14, 2019

The same Meta.Background could be used by multiple instances of background
actors, and so should not be disposed when the actor using it is destroyed.

Instead of calling `run_dispose` directly on it, just nullify the reference
on destroy method, leaving the job of doing the proper disposition to the
gabage collector that keeps the proper reference count on the Meta.Background.

Fixes #501

!558