applemedia/avsample: Fix racy cleanup of CA layer

applemedia/avsample: Fix racy cleanup of CA layer

The block that is dispatched async to the main thread assumed the
wrapping GstAvSampleVideoSink to be alive. However, at the time of
the block execution the GstObject instance that is deferenced to access
the CA layer might already be freed, which caused occasional crashes.
Instead, we now only pass the CoreAnimation layer that needs to be
released to the block. We use __block to make sure the block is not
increasing the refcount of the CA layer again on its own.

https://bugzilla.gnome.org/show_bug.cgi?id=753081