This is part of VK_EXT_mutable_descriptor_type but we never did anything
with it. Since we use local memory for descriptor sets, copying from
them means reading VRAM through a WC map and it's pretty expensive.
Using malloc() for HOST_ONLY should be a nice perf boost for things
which give us the hint.
This massively improves the performance Dragon Age: The Veilguard,
taking it from 7 FPS to 25 FPS on an RTX 4060.
Closes: https://gitlab.freedesktop.org/mesa/mesa/-/issues/12622
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/33571>
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