MemScope::System has to synchronize with everything in the system,
including across PCIe so it's horribly slow. MemScope::GPU, on the
other hand, only has to synchronize within the GPU. This is way faster
and still satisfies all of Vulkan's requirements because Vulkan never
allows CPU<->GPU access without full semaphores and barriers.
Reviewed-by: Mel Henning <drawoc@darkrefraction.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/33649>
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