The guard that refuses push descriptors on MoltenVK keyed on the
VK_KHR_portability_subset device extension, but that extension is not
enumerated for every device: the headless device the draw replay and the
device tests create took the raw driver limit, so replays on macOS read
guest storage buffers past their first bytes as zero (MoltenVK reports a
zero OpArrayLength for buffers bound through push descriptors).
The limit now comes from one policy both the presenter and the headless
device use, and macOS, whose only driver is MoltenVK, pushes none
regardless of which extensions are listed.
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