`olShutDown` was not properly calling deinit on the platforms, resulting
in random segfaults on AMD devices.
As part of this, `olInit` and `olShutDown` now alloc and free the
offload context rather than it being static. This
allows `olShutDown` to be called within a destructor of a static object
(like the tests do) without having to worry about destructor ordering.
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