How ChooseRealtimeVendorForAvatar instantiates and judges a vendor; each caller has its own resolution shape.
Optional
Whether another vendor's resolution may stand in for the first one; absent accepts any.
The vendor's resolution (its driver on its key), or null when it has no key or no driver.
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Whether the resolution renders an avatar (RealtimeModelShowsAvatar).
How ChooseRealtimeVendorForAvatar instantiates and judges a vendor; each caller has its own resolution shape.