Finalizers should be cleared within the Destroy() methods

[justinfx]

Various types which wrap the C API pointers (MagickWand, DrawinWand, ...) create finalizers to ensure that the C memory is at least freed at some point later when GC runs. Users also can free the memory immediately by calling the Destroy() method on the object. While the call to Destroy is idempotent, we should make sure to clear the finalizer in the Destroy method to reduce the need for the GC to run finalizers that don't do anything.

runtime.SetFinalizer(mw, nil)