Wrapped pointers don't get cleaned up in some cases

[turboladen]

When, say, a OGR::Point (that wraps a FFI::Pointer in @c_pointer) is garbage collected, that FFI::Pointer needs to be released using OGR_G_DestroyGeometry().

We can do this using an FFI::AutoPointer and an RBS interface of HasAutoPointer[P].

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Originally posted by Steve Loveless (@turboladen) in #27 (comment)