For a resource object that can be retained by descriptors beyond its lifetime,
release memory resources when the object is destroyed by the app. This includes
objects of type MVKBuffer, MVKBufferView, MVKImageView, and MVKSampler.
When the app destroys an MVKBuffer, also detach from the MVKDeviceMemory,
to fix a potential race condition when the app updates the descriptor on
one thread while also freeing the MVKDeviceMemory on another thread.
MVKImageView guard against detached planes while in descriptor.
Add comment to clarify how destroy() is called from release().
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