It is optional and is needed only when a layer has physical device
extensions that may be unknown to the loader.
This simplifies the layer a bit, but more importantly, it works around a
bug in the loader when there is another layer in the layer chain that
wraps VkInstance.
Reviewed-by: Dave Airlie <airlied@redhat.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/16307>
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